Quick update: IT WORKS NOW!!
Suddenly, after restarting Dorico, this dropdown was filled with all the frequencies one would expect, and after selecting 44.1 kHz everything was fine: NotePerformer was filled with all the instruments and I could finally hear audio output.
Shout out and a big thanks to @Ulf for his tireless work, even offering a 1-on-1 session to fix this issue!
If you ever plan on visiting Vienna, DM me for a beer or two 