THEY EXIST!
26.06.2022: I have found good EU keyswitch vendors!
It took a while and some luck, but I have finally found a few keyboard switch vendors that are based in Germany (and some in the EU)!
The first one, keyb.no, was recommended to me by a Scandinavian friend. They have a good selection of switches, they make their own lube and lubrication products and they also have a few keyboards, keycaps and other accessories in stock. The problem for me personally is that they are (as the URL suggests) based in Norway, which doesn't really translate well to shipping costs to Germany.
The second one I found was candykeys.com who are based in Munich, which makes shipping around Germany significantly cheaper. Their switch selection is massive, they have lots of options for (even ISO-DE and other EU variant) keycaps (though they're also out of stock of most of them), a good variety of keyboards in various form factors and a bunch of accessories, pcbs and tools.
The third one I found was keygem.store who also have good shipping rates to Germany, all kinds of switches and also lots of other parts, with quite a few premium options available.
Well then, onto designing the rest of the keyboard. Still not sure if I actually wanna go down the route of entirely DIY-ing this thing or if I wanna get me a kit. Though I did settle on getting me Gateron Black Ink V2s as switches.