This answer is too late for your trip but might be useful for others.
I've just come back from a 3-week trip (September 2023) driving in southern Germany and rented a Model 3 from Hertz at Frankfurt airport. There's no problem with charging the car during my trip. At most superchargers, there were only my car or one or two others. I never had to wait for a spot. The charging fees are accrued to my rental and were paid when I returned the car. The cost of charging is surprisingly low. It cost me on average less than $50 a day for both rental fee and charging fee (extra insurance declined). I drove less than 4000 km while the allowance was more than 5000 km so I didn't have to pay for extra miles.
Driving in Germany was a pleasure: roads extremely well maintained, nary a pothole. Drivers disciplined. I didn't have to worry about someone trying to sneak past me on my right. Highway on ramps and off ramps well designed and safe unlike the crazy few yards I get for merging at some places in California.
The only 2 minor things: garage spaces tend to be narrower than those in the US and driving in old cities sometimes can feel like driving ON busy sidewalks.
