Success! Everything works like a charm!
I did exactly what @nephros wrote from step 1b (chum was yet installed) to step 8 using a Nextcloud account. Naturally, with tears in my eyes, I had to change nemo into defaultuser, but the rest was C+P and worked at the first try! It’s a little bit slow and it doesn’t refresh directory info realtime, but in general it does exactly what I want!
Thank you so much @nephros and @poetaster