And some updates here. I eventually had and electric line brought from the meter to my garage. So I decided to test some old Powerline adapters I had in a drawer since 10 years ago, and they work amazingly well. Out of the 30 Mbps I have with my FTTC data link at home, I get almost 15Mbps in the garage. I have not tested LAN performances yet, but Internet access is pretty good all considered.

These are old 85Mbps Powerline adapters. I guess newer and better adapters may even improve the already good results.

I live at floor 2 (EU, which means floor 3 in the US) in an apt building, so to reach my garage I go all the wt through floor 1, ground floor, basement, and 20 more meters to reach my garage from the meter. Performance is way better than I hoped for.