The Rick Steves itineraries do involve a lot of moving around and you may well end up seeing more of Scotland through the windscreen rather than actually experiencing Scotland. Unfortunately many people end up slavishly following them, regardless of whether they are right for them.
Your idea of choosing bases for several nights and exploring from them is the right way to go.
I actually prefer
DK Eyewitness Scotland as a travel guide. It has a lot of pictures and the maps are good enough to get started with planning. It covers many areas ignore by the other guide books.
Once I’ve decided on an area I an to visit, I go to the
Undiscovered Scotland website. This is a wealth of ideas. start with the relevant map page and then follow the links to text pages with photos and information.
Don’t underestimate driving times. Scotland may look a fairly small area compared with the states, but distances will take a lot longer to cover than you might expect. If using google maps it is recommended to add 25% to their driving times and then add on time for stops. Off the motorways, roads are often narrow and can be very slow.
August is the height of the tourist season and many places will already be fully booked up - this particularly applies to places like Skye and Mull, so I've not included them in these suggestions, although it would be possible to see some of them on a long day tour. I've also discounted Orkney too along with shetland and Western Isles. (more wonderful places for another visit!)
Starting in Glasgow - spend 2or 3 days in Glasgow to recover from jet lag. There’s so much to see and do in the city - cathedral and many world class museums, especially the Burrell Collection. You won’t need a car in Glasgow.
Pick up a hire car from the airport. Arnold Clarke get consistently good reviews or Celtic Legend.
Head to either Oban or Fort William for your first base. Then head across towards Inverness, but rather than stopping in Inverness itself, think Dingwall, Beauly or Strathpeffer to the north and west OR Nairn to the east.
My final base would be St Andrews or one of the Fife Fishing villages.
I would suggest 4-5 nights for each of these places. (Use Undiscovered Scotland for isdeas for each place).
You are wanting to finish in Edinburgh BUT be warned Edinburgh is always busy in August. Accommodation is at a premium, probably fully booked and prices are sky high. The Tattoo finishes 26th August and the Festival on 28th. You may have problems finding somewhere suitable in Edinburgh itself before then. Linlithgow is a possible or else head for somewhere to the south in the Borders - possibly Melrose for the rest of the stay.
If you do stop in Edinburgh itself, you won't need a car so drop the car off at the airport and use public transport.
I did look at trying to include a loop round the top of Scotland on what is marketed as Route 500 but that would involve a lot of 2 night stays and there is limited accommodation up there and (like Skye) it is likely to be fully booked in August. I've tried to think of places where you should be able to find accommodation without too many problems.
Use this very much as a taster trip. I'm sure you will want to be back and there are so many more wonderful places to explore...