Ardrossan to Brodick is a heavily travelled route, so unless you are taking the first ferry of the day or the last ferry of the day, you'd need only a month's advance booking at the most.
Lochranza to Claonaig is a "show up and go" ferry. You can't make a reservation, although you can pay ahead of time.
The route from Oban to Craignure is also heavily travelled, so again, unless you're taking the first sailing of the day or the last sailing of the day, a month's advance reservation would be all you need.
I'd recommend Tobermory to Kilchoan above Fishnish to Lochaline. The drive once you reach the mainland is stunning. This is a small ferry, but the route is lightly travelled. Last time we sailed, it was a "show up and go," although we paid ahead. That may have changed since 2018. However, the Fishnish to Lochaline ferry runs about every 45 minutes in summer, so it's most likely a "show up and go" route as well. If you take that ferry, you have the option of driving to Mallaig by way of Loch Linnhe, or heading west from Strontian and joining the Kilchoan to Lochailort road. Both routes are a bit longer than if you take the Tobermory to Kilchoan ferry.
Mallaig to Armadale should be reserved about two months ahead. Same with Uig to Tarbert and Stornoway to Ullapool.
If you change your mind about any of these ferry crossings after you've made your reservations, CalMac charges 10 pounds to cancel or change.
In addition to the work at the Uig terminal, the ferries have been known to break down. When that happens, CalMac will bring in a ferry from another route. Sometimes it's a smaller ferry; sometimes it's a larger ferry. That's why it's always good to reserve as far in advance as you can.
To me, ferry travel is one of the best things about travelling in Scotland. There's something about the sea air, the announcements in Gaelic, and the chance of seeing dolphins or whales, that really appeals to me. Even though the islands are as modern as the rest of Scotland, there's that feeling of having stepped back in time.
I don't know how much time you have on Harris, but if you have a spare day, a sail out to St. Kilda would be a journey you'll never forget! The boats leave from Leverburgh, and it's three hours each way, plus about six hours on Hirta.
I know that you are going to truly love your holiday in Scotland! My very best wishes for your travels.