Hi, SS,
This past summer, I had a 28 day rental with Avis, out of Aberdeen airport. I had contacted CelticLegend for a quote, and they came in at over $200.00 more than Avis. Plus Arnold Clark was off site, which would have added unnecessary down time to our holiday.
In 2022, I hired a car from Avis at GLA for 30 days. I had no problems with it, other than the fact that the car had a 1000 cc engine, which made going uphill a challenge! Avis is very reasonable, as long as you reserve far enough in advance.
30 days, by the way, is the maximum amount of time allowed to use your credit card coverage for all damages to your hired car. That will save you a lot on CDW if you decide to use your card for that purpose.
Personally, I would much rather hire the car out of Glasgow, as opposed to Inverness. It would give you the opportunity to visit places along your route, and to be able to stop to eat wherever you like. Unless you have driven in Scotland before, I would recommend heading back out to the airport to pick up your car. It's easier to get out of town from the airport than it is from center city.
Sounds as though you have a great holiday planned! However, you may be limiting yourselves by basing in Inverness for the entire time, especially if you plan to do a lot of hiking. Had you thought about basing in Inverness for two weeks, then somewhere else closer to hiking sites for the following two weeks?
Best wishes, and safe travels!
Mike (Auchterless)