I have owned three teslas myself and have rented in both Italy and France as well as numerous places in NA. Thus far I haven't rented a Tesla from Sixct, although I am quite experienced with Sixt with ICE and have never had any problems at all with them.

Every EU Tesla you'll find with Sixt has the standard EU CCS standard plus, so you can use it at any Supercharger, destination charger of any other charger. Charging stations of one type or other are ubiquitous all over. Several years ago I drove from Milan to Dubrovnik, then to Vienna and back to Milan, Superchargers everywhere. The easy way for Tesla is to use their 'find us' feature on Tesla.com and select your geography, then eliminate Store and Service. You'll then sell you can pretty much go anywhere in Europe. Southeastern Europe is a bit sparse but otherwise there is zero issue.

The charging procedure for Superchargers may have changed with Sixt. I have made a corporate inquiry and will pst it when I receive it.
Otherwise I find this to be the easiest way to charge non-Tesla in the EU https://chargemap.com there are also numerous country-specific networks too. Nearly all, EU wide, allow credit card use for first time use, but usually it's simpler with a login. Plugshare is useful almost everywhere in Europe, including even Bulgaria, Greece and Turkey. I typically use Plugsahe to find out what network dominates in thea area I need and, if not Tesla, find out if Chargmap has coverage, if not I usually sign up for the locally dominant option.

This may sound complicated but once you learn a bit it becomes second nature. By far the easiest thing is to use Tesla everywhere. Major hint: When traveling check to find hotels and/or restaurants that have charging. Most are free. I have managed to not even use a charge on most days, but recharging free overnight. Nobody talks about that but the magic of trip in a Tesla or other longer range EV is that you rarely use charging during the day, but charge overnight. I usually check on the Tesla site for Destination Chargers, and stay in hotels that have them. Plugshare can find non-Tesla options too but with those you need to check the connection type because electrical outlets are NOT standard across Europe and a rental car will not have a wide variety of adapters.