I think a lot of posters are reading into this too much. OP says they have a long term EU residence card, which based on this site sounds like OP would be able to work in Germany.
OP - I went to college in the UK. I worked for a couple of barns while I was there. Nobody famous, just local places with nice horses. Never any paperwork and cash payment (~15 years ago now ). I sure worked hard but I got to ride and experience so much more than I ever had and ever will in the US. Iād do it again in a heartbeat so my vote is go for it.
Quick edit to add - it sounds like OP was initially on a student visa then stayed the length of time required in their country to get the long term residency card. If in doubt OP can contact the proper government authority to confirm.