i am doing it nealry the same, but with SeaFile. I could suggest following :
--dir-cache-time 1h ===> 192H (or more) You will save a lot API requests & file listing will be faster
--allow-other ===> Is okay, don't know why, but without it doesn't work.
--vfs-cache-mode writes ===> full - FTP offers resume, so the file will be cached one time for Xh and you save again API requests + in case somebody mounts this FTP as a share and is trying to stream you prevent additional API req.
--no-seek ===> Again, if somebody mounts it and try to stream
--vfs-cache-max-size XXXGB ===> choose a good size, everythings' cached, saves API req.