Too much moving around?

Take the train to Stirling on your day of arrival and visit Edinburgh at the end of your trip.

If you are flying into Edinburgh pickup car there and drive North to selected locations and turn around and go back to Edinburgh via Stirling and drop off car at airport, shuttle to Edinburgh, spend the end of the trip there, since you really don't need a car in the City. We picked up a car upon arrival in London the 2nd time and drove to our 1st destination which was less then 1 hr away. It depends on if you are not afraid to drive on the left, just follow the traffic flow. Rest as much as you can on the plane, and you'll probably be all right. This depends on you have experience with long distance driving. It helps if you study maps ahead of time. Driving in Scotland is not too hard unless you drive off pavement. It may be easier for some instead of hauling your luggage on public transit, Training to Stirling for 1st night could be an option, although your choice of car rentals is more limited than the airport. Be sure to reserve ahead of time.
PS Spring may be late in April/May. Wear layers and water- resistant jacket and footwear and rainhats. You can bring your trekking poles if you can pack them in your luggage.

Do not pick up a car on the day of arrival after a night of no sleep. Especially since driving is on the opposite side of the road and the steering wheel is also on the other side. There is no room for error here.

Spending your first 3 nights in Edinburgh makes sense. Take a taxi or uber to your hotel in the city. Pick up your rental car at the airport when you leave Edinburgh so that you don't have to drive in the city.

You can visit Stirling Castle on the day you leave Edinburgh and before you get to Glen Coe. There is no need to spend 2 nights in Stirling. I would use those 2 nights to add to Skye. We spent 4 nights on Skye and followed this roadtrip itinerary
https://www.ontheluce.com/isle-of-skye-itinerary/. We saw all the major highlights of Skye. Her itinerary is very well organized, and you don't do any backtracking.

When you are in Glen Coe, which is stunningly beautiful, I suggest taking a drive down Glen Etive Road. This is a one-way 12-mile long road that follows the Etive River and ends at the mouth of Loch Etive. Another highlight of our trip. The mountains were covered in rhododendrons when we were there in June. April/May might be a bit early for rhododendrons. You will be surrounded by lush greenery, the river, the lake, and the mountains.
https://www.ontheluce.com/glen-etive-road-scotland/
Oh, did I tell you a scene from the Bond movie, Skyfall, was filmed here?

Instead of staying in Inverness, I suggest staying in Nairn, a very pretty seaside town about 30 minutes from Inverness.

We were trying to visit Scotland this past May, however we were having difficulties finding reasonably priced accommodations. I had no idea Scotland was such a popular travel destination.

So I just suggest you make room arrangements as early as possible.

We ended up having to go to Plan B--a cruise out of Rome around the Greek islands, Amalfi, Sicily and back to Rome (Civitavecchia.)

I would also suggest you don’t need two nights in Stirling, unless you are keen on seeing the town and not just the castle. Inverness does not need two nights, either, but I did make a one night stop between Inverness and Skye. The sights around Inverness can easily be seen on the drive into, or out of, town. There is not much in Glencoe unless you are planning to hike, and it might still be pretty cold and wet for that in April, but the drive is pretty. Definitely add more time on Skye. If you are flying in and out of Edinburgh then it might make sense to switch the days around, spend the first night there to recover from the flight then head for Sterling, and spend your last few nights in Edinburgh before flying home. I hate being in a rush to return a car and jump on a plane. I worry that the car will break down or I will get caught in traffic. You could also just go straight to Sterling the day you land if you arrive early enough. On your return to Edinburgh, if you have enough time, you might even take the train to Glasgow for a day. It’s worth the trip.

I'd definitely want a waterproof, not water-resistant, rain jacket. Western Scotland gets a great deal of rain. It's likely to be chilly enough in April that getting soaked to the skin will be unpleasant. Even in June, I was glad to be wearing merino wool long johns under my nylon hiking pants, because my legs did get quite wet on occasion--and I wasn't doing any rural hiking, just walking around outdoors. For an April trip I'd be inclined to take a pair of rain pants, but I admit to being very cold-natured.

You could go straight to Stirling on the day of arrival. It is a short, fast and frequent journey by train from Edinburgh- Waverley or Haymarket.
It doesn't matter what time you land, as trains run until close to midnight.

With such a good train service there is zero reason why you can't base your time for Edinburgh in Stirling. Far better hotel prices, even after adding the train fares on.

I'm not at all sure that Glencoe (1 word) merits two nights. The second night there would be better distributed elsewhere.

I like the spending some time in Edinburgh upon arrival to get over jet lag and then again just prior to departure. The Kelpies and the Falkirk Wheel are easy stops en route to Stirling. We also stopped to view the impressive Forth Bridge. If you’re going to visit places like Culloden, Ft. George, or Clava Cairns, then two nights in Inverness makes sense. Since you’ll have a rental car, if you like smaller castles consider Doune castle. It’s only 8 miles from Stirling and was used extensively is the first couple seasons of Outlander. As an FYI, other than at Stirring Castle, the parking machines in Stirling only accepted coins. That was September 2022.

By history, do you mean Neolithic or middle ages or modern? Natural history or art museums?
By outdoors, do you mean hikes (long or short), drives, mountains, coastlines, birding, wildlife?

I don't think since the advent of online reservations you can "ad lib" any longer unless you're prepared to sleep in the car. Our first trip (1997) was reservation-free and we didn't really have more than a vague itinerary. We just called B&Bs as we pulled into towns. Our 2024 trip is mapped out to the nearest 15 minutes and everything was booked three months ago. Answer the questions, and you'll get a lot more suggestions.

Here's my personal thoughts based on our preferences:

I find one-way car fees very irritating, so I'd rent the car in Edinburgh and drive to Stirling - it's not a long trip and driving will probably be faster than making public connections.

We would much rather poke around an 11th century castle ruin than restored "operating" castle. Having been to Stirling and Edinburgh Castles, I wouldn't do both (or either, if I could rewind).

One night in Stirling should be plenty and you might want to consider Doune Castle instead of Stirling.

We found Inverness town to be pretty dull (except for the music) but Culloden and Clava Cairns are both interesting. Staying in Nairn might be a good suggestion (I won't know until next year).

I'd move one of your spare nights to Skye, as Trotternish Peninsula should consume more than a day if you do any hiking and there's the other two-thirds of Skye.

Flying to Orkney (or Shetland) from Edinburgh or Inverness is another complication, especially if Neolithic things are of interest. Flying from Edinburgh lets you dump the rental car for good.

I always try to cram something into the long-haul flight arrival day (like a connecting flight to Orkney) since I'd rather nap on a plane or train and see more. Similarly, we'd try to arrive at EDI for our flight from another spot (like Stirling or Anstruther) rather than spend the last night in Edinburgh. Really depends on your flight times and how sleep deprived you are.

Whatever you end up choosing, you'll probably want to come back to see more.

I agree with David about accommodations. I started booking rooms for my May 2023 stay in July and August of 2022. And 2024 will be even worse. Scotland recently imposed much higher taxes on short-term rentals, and many B&B owners have decided that it is not worth the higher cost to rent out rooms or self-catering places. This, of course, means there will be fewer accommodations available to rent. So it would definitely be advantageous to book as soon as possible, especially on Skye.

Depending on what time you arrive in Glen Coe, I think you do need 2 nights, especially if you want to do some walking/hiking. We arrived close to dinner time. We stayed 2 nights, and we spent our first full day there taking our time driving down the Glen Etive Road, stopping frequently and taking photos. We stopped at viewpoints quite frequently on the main road for photo ops. We also took the chairlift at the Glen Coe Mountain Resort.

Depending on what you want to see in the Inverness area, you do need 2 nights and possibly a 3rd. As I said in my earlier post, we stayed in Nairn at the Sunny Brae B&B and loved our stay there. Inverness is a busy commercial center; Nairn is a much prettier seaside village. From Nairn, we visited Elgin Cathedral, Cawdor Castle, Culloden Battlefield, and the Glen Moray Distillery for a whisky tasting. We visited Urquhart Castle on our way to Skye from Nairn.

I think Braemar (or nearby Ballater) is worthy of more than a one night stop. For a small village there is a surprising amount going on there and in the immediate area. There's a very proactive tourism group in the village and they keep the website up to date.

https://braemarscotland.co.uk/

This page of the Walk Highlands website will deal with walking in the area

https://www.walkhighlands.co.uk/cairngorms/braemar.shtml

https://www.walkhighlands.co.uk/cairngorms/ballater.shtml

Braemar Castle has a long and interesting history and with luck might well be open in time for your visit. It's been closed for the last couple of years while major renovations take place and I understand these took longer than expected (don't they always?) which is why it didn't open in 2023.

https://www.braemarcastle.co.uk/

Wherever you choose to go please book accommodation in advance. Scotland is definitely a popular destination thanks to the Outlander Effect and as already mentioned, the short term rental licenses being brought in by the government are going to have an impact on the amount of accommodation available going forward.

Thanks for the advice and comments