Well, just a few things I'd like to note, none of which will really actually help you I'm afraid. But, still. The reviews will by and large be partial because the average person writing a review is more interested in relaying an emotion than what an experience to expect, the average reviewer is not a journalist writing an impartial piece on the topic, and vice versa, even if an average journalist would write a review of their rental experience, it would likely not be impartial.

By extension as well, the reviews typically represent one side of the story, which makes it very difficult to assess what actually went on, when something went wrong. There are a lot of stories out there about fraudulent damage claims, excessive fuel charges, etc. but it's very tricky to correlate these to company practice, it may just be bad employees, or bad decision making, or an unfortunate sequence of events. Or, of course, it may be a blatant scam. The point is, without more information it's difficult to know for sure. The reviews do not really represent the actual experience

So then, it's best to just protect yourself from these kinds of things insofar as you can. As I mentioned before, you can eliminate mostly any damage claims by making a careful record of the way you receive the car, and ensuring any prior damage you notice is listed on the sheet. So at least take the time to do a walk around and check for the obvious spots of damage + document them (pictures/video). A good indicator (I find) is if the damage sheet reports damage that you do not see straight away yourself - sure it means they take a careful look at the vehicle when you return it, but it also means they will not neglect mentioning/hide damage to claim you caused it. Do the walk around again when you return the vehicle, and document that (to be certain), alternatively, get someone from the rental agency to go over the car with you and ask for a copy of their findings (not sure if you can, actually).

A second part is insurance coverage. In Europe, renting through a European Sixt site, I think when you rent a vehicle you typically get a full covered rental (liability and collision damage insurance), this should be noted on the rental form (reservation and/or receipt). I think for an additional cost you can get your own liability to damage to the vehicle reduced to 0. In addition Sixt offers supplemental insurance to also cover flat tires or replacements thereof, under their roadside assistance package. All of this should reduce any impact a mishap may have. At a cost. Not necessarily advocating it, but it's there, and because of European laws (as far as I understand) you already get a quite good base coverage.

Other car rental agencies. Or perhaps ride rental platforms like Turo, though I don't think they're present in Europe.

It surprises me to hear you can't get Mercedes Frankfurt to return your e-mail or call. Are you inquiring with them about renting a vehicle? I'm not sure they do, they would probably offer you a test drive free of charge, though. Depending on the dealer this can mean you get the vehicle for (a portion of) a day, to get your own impressions.