Do you have specific sites or places which you plan to visit while staying in Oban? Oban is usually described as a place to go to get to other places, or to paraphrase the Book Voice in How to Succeed in Business Without Really Trying, "...a place out of which you must get". Oban can be used as a staging place to get to the nearby islands, or to visit Kilmartin Glen.
With eight days on the ground in Scotland, including one day in Stirling and one day in Edinburgh, you have six days left to enjoy Scotland's beauty. If you're heading north to Inverness prior to Oban, your best bet would be to take the A9 north from Stirling, as you're somewhat limited for time. You could stop in Aviemore and visit the western section of Cairngorm National Park. If you drive straight through, however, it would take about three hours at a leisurely pace from Stirling to Inverness.
Once you've reached Inverness, there are many choices of day trips which you can take, in addition to places to visit in and around the city. Personally, I'd recommend driving out to Lochcarron, then up through Glen Torridon, with a possible side trip over the Bealach-na-ba. Another choice would be a day trip to Gairloch and Ullapool, or a drive along the Moray coast, to visit some of the fishing villages. But the choices are yours. Check more than one guidebook. Look for travel videos on YouTube. There are some really good ones out there.
You had a great idea about staying in Stirling and visiting Edinburgh from there. The money that you save in lodging and parking will more than pay for the train fare. With just one day in Edinburgh, you need to plan your visit really well.
Good luck with your plans! Please ask if you have more questions. The folks who post on this forum are people who love Scotland, and are happy to share that love with you!
A' the best,