Hey guys, haven't noticed any new answers, so thanks
My phone is not financed (paid cash €€€) but also bought at a retail store, not from Apple. I am indeed from Germany and thought it would be a good thing to get the protection plan. I now realize it's just an extension of the base warranty with free telephone support etc., which I don't need anyway. I should have done my research earlier, that's for sure.
Anyway. I made a vinyl skin for the back and got a new, semi transparent case, and now it looks pretty again ;-)
Not sure if I'll keep the phone for testing (working as a dev) or if I'll sell it for cheap next year when the iPhone 12 is out.
Found some guys that replace the back glass for less than 640€, but I guess it involves manually breaking away the remaining glass with force, and I don't think I want to risk that. Found some YT videos about some shops removing the back glass with a laser, but I guess there aren't any of these dudes anywhere near my location. Maybe next year.