A web site to check out is by a Scottish traveller who does only public transport trips, https://www.thechaoticscot.com/home/scotland-by-public-transport/
I had a thought but she has it written better than I was coming up with while trying to compose .. one thing is to figure some locations to use as bases. She also recommends staying 3 nights in those bases, which I will definitely recommend. My 2025 trip had mostly 2 night stays, so a 3-night stay is the one that felt the most(!) restful of the whole trip.
The Chaotic Scot gives suggestions for bases and they do look great. If you have enough time for browsing, get to know the train route maps. Maybe you'll have something else you want to try, https://www.scotrail.co.uk/plan-your-journey/our-routes
Another site I read a lot is Wayfaring Kiwi. She has a lot of great info all over the site, and I see the shop also has a public transport item, https://shop.wayfaringkiwi.com/
Historic Environment Scotland has a section for sights to see without a car on this page, https://www.historicenvironment.scot/visit/things-to-see-and-do/
Visit Scotland has a page for travelling around by train, https://www.visitscotland.com/travel-planning/railway
As you bookmark sites to see, perhaps it's not every single thing, but all the pages I remember will have info and if a location is available by public transport, the info will be included.
If something you desire is a bit out of reach, for day tours, you are bound to see recommendations for Rabbies, https://www.rabbies.com/ I have enjoyed quite a few of those, both days tours and a few multi-day. Last year, I used a driver guide to get from the airport on my first day, combining a transfer with sight seeing. The Wayfaring Kiwi and her husband do tours, but Craig was unavailable the day I needed, https://kiwiandhaggistours.com/ I found a driver guide through the Scottish Tourist Guides Association, https://www.stga.co.uk/
Happy researching. Cheers.