I'm a long time Opus user but only recently started to use the File Collection feature.
I would like to add one suggestion here.
When a file is moved, it vanishes from your collection without any warning. This does not happen when its renamed. The hard link in the collection gets renamed as well but if you eventually create a nested folder and try to organize your files in a better structure, it will mess up your file collection as obviously Opus cannot find the file anymore.
It would be great if this could be somehow avoided in the future. Maybe Opus should look for the file in at least one nested folder before giving up or display a message so you can properly update its path.