I think it'd be much simpler and much safer to rent for the maximum and possibly return early, and just realize that depending on how early you return it and also depending on the rental agency you may or may not get any refund. Rates are often "weekly" rates past a certain number of days, and in that case any shortening from 7 days to that number of days may not change the rate.) But the rental company won't expect you back before the maximum, so if you come back early that's good news for them.

On the other hand, if you rent for the minimum, and then there turn out to be problems with extending it, you could face steep fines for not returning on time. Don't assume you can necessarily extend it. The rental company will be expecting you back at the minimum days (if that's what you rent for), and if you come back late (or just ask to extend it) that's bad news for them (especially if they don't have plenty of extra stock).

Lots of rental companies are short on stock right now (because they sold off a bunch at the start of the pandemic and haven't been able to re-stock due to the car-makers' chip shortages), and you want to keep that in mind with all rental decisions you make.