Can't write to OpenWrt mounted disk from Mac's qBittorrent

Probably ksmbd... here's some info on how to do it with that:

(you'll still need to install more stuff, though).

If you don't have enough space, you can try to remove packages you aren't using, or you might want/need to create your own custom image. Alternatively, extroot could be used... but aside from that, you may be out of luck... newer hardware with more space may be necessary.