My number one rule of itinerary planning is to decide which 2 or 3 sites/locations are important to me and build around that. You mention sightseeing. Which sights interest you?
I use public transport when I travel and found it was fairly easy to make it around the parts of Scotland I visited by bus and train.
My past itinerary:
I traveled with my sister this trip and we were coming North from Hadrian’s Wall Country, so we started Scotland with a few hours in Glasgow between trains before we went to an overnight stop along Loch Lomond where we planned a day hike on the West Highland Way. Perhaps I had too high of expectations here or am simply spoiled by living in the location where I do, but this stage was a bit of a disappointment. I would not stop here again. We continued on by train.
Next was three nights in Oban (two would have been enough) where we went on the West Coast Three Island tour which was one of our top three interests. Loved seeing the Puffins on Staffa! Thoroughly enjoyed the islands. Next was the train to Stirling. The goal here was really the Falkirk Wheel, but we found we very much enjoyed Stirling Castle as well. The Falkirk Wheel was one of our top interests. It probably wouldn’t be on everyone’s radar, but that is why thinking of your personal interests when itinerary travel planning is so important. On to Edinburgh where we spent one day exploring usual sites. The next day we had different interests so my sister explored textile options and I took a bus to St Andrews where I had a blast playing the Himalayas, walking the beach, etc. Our trip ended here and we flew home.
I saw only the tiniest section of Scotland, but I focused on locations that were key to me and didn’t worry about trying to see everything.