Hello, I found this thread when looking for others who have had delayed Tesla supercharging when renting from Sixt in Germany. I can share my experiences here to help several of the questions below:
I rented a Tesla 3 times in 2023, in May, June and September and drove approximately 4800 km, 4200 km and 3900 km respectively.
The first and last times were Model Y (long range first and then performance) and the second time was a Model 3.
P.S. - There was a huge difference between Long Range and Performance when I could get to max speeds on the Autobahn, the long range shook at speeds above 190 kph, but the performance was very smooth.

There is zero problem with finding places to charge when driving long distances and most of the time the hotel you stay at has one or two level 2 chargers that can get you topped up for the next days travels (though they can be occupied and the owner may not move it after theirs is charged).
A lot of businesses also have level 2 chargers, so if you are visiting a company they may have an option for you to charge while you are there for meetings, etc.
The lack of issue is due to the vast network of Superchargers that the car will navigate you to as long as you enter in your destinations for the day or multi-day travels on the screen.
And there is no need to interface with the supercharger for connection/payment/etc. the chargers just work and you just take the connector from the supercharger and plug it into the car and it charges.

The only issue I experienced with Supercharging was that on my second rental in the Model 3, Sixt neglected to set up the car to tie it to my rental and I couldn't supercharge on Tesla's network for the first day. They were late in delivering me the car after I landed in STR and probably forgot to set this up in their rush to get it to me. After it was resolved there was no issue for the rest of the rental. I had to stop at the nearest Sixt location on my several hundred km route and it prevented me from getting to a meeting for about 3 hours until it was fixed.
So, make sure to check that there are no errors which might indicate that there is no payment method tied to supercharging before you leave the rental lot. If there is no warning symbol on the top right, at least try to navigate to somewhere very far away (example: if you are picking it up in STR try to navigate to CDG and it should give a warning that you will need to supercharge and there is no payment method set up to do this and should display in big red letters).

However, I would not have rented any other electric car than a Tesla because the high speed charger network is not nearly as widespread and reliable nor the tight integration with the navigation to them along your programmed route.
This has likely improved since then and Tesla, I believe, has opened up their chargers for other brands and since the connector plug is the same in Europe for all EVs this isn't as much of an issue as it is in the USA.
I also think the fee per kWh is more if you charge a non-Tesla on their network.

As far as return amount of charge, as others have said, there was a 5-7 euro fee I paid to be able to return it with any amount of charge over 10%, I think it was.
As I mentioned at the beginning, though, Sixt is only just now billing me for the first of these supercharger uses and still only for the ones in Germany, though I drove into Italy and Austria. So, I'm trying to fight them about it, but I've heard that they can continue invoicing for up to 3 years from the date of service, which seems excessive. Thus, the supercharging is not free, just very delayed.

Oh, one more tip, use Waze when driving so you see the speed cameras and potentially the pop up ones. It saved me many euros many times. You don't need to use it for navigating, just keep it open and set so you can hear the warnings. Sometimes you only have about 10 seconds of warning before a speed camera alert comes up. Works in all countries I drove through in Europe (Germany, Italy and Austria).