It sounds fine but if you want to get a taste of Sweden that most tourists don't, I have a suggestion. (I have driven this and I am a senior citizen.)
Another option is take the train to Malmö and pick up a rental car to drive to Kalmar.
Drive along the southern coast of Sweden on E6/E22 (Skåne) hitting Ystad (of Kurt Wallander films) for a few hours. From there you could stop at Ales Stenar (a Viking monument) for an hour. When I was there there were hang gliders flying overhead as this area is on a cliff (which has a path down to the sea). There is parking at Kåseberga harbor. Take Öst Kustvägen towards Simrishamn. There are sandy beaches at Sandhammaren, which is a nature reserve not far from Ales Stenar. Continue past Simrishamn to Kivik.
The area around Simrishamn is known as Österlen and is one of those hidden gems, in my opinion. Kivik is famous for its apple orchards. Kiviks Musteri makes Swedish apple juice, cider, and other items. North of Kivik are secluded sandy beaches and pine forests. (Friseboda has parking). They're accessible and public. You will need to get a bit off the beaten path from Highway 118 to access them. From Åhus continue on 118 to E22 and head to Kalmar.
It's a 200-mile drive from Malmö to Kalmar. It's safe and easy but lovely. You could stop anywhere along the way to have lunch or coffee. There should be plenty of parking opportunities. Drop off the car in Kalmar. Continue by train to Stockholm.
Swedish car rental companies offer both manual and automatic cars. I read that you don't need an international driving permit here but it might be good to have one. I always purchase additional insurance to cover the deductible in case of an accident (or even a scratch).
Late August is a good time of year. The weather is pleasant. Most touristy places are still open even though most Swedes have gone back to work and back to school. You will still have several hours of light so the days are still long enough to enjoy them.
I suggest you cut down on Oslo. The Viking Museum is closed until 2027 and there's just not much else there. One night is enough.