Jumbled deltions and unsynced directories across multiple devices; any way to set sync to be only additive?

I have deployed Seafile in my home as a sync/backup solution for several music/media production computers, and though I have made a lot of mistakes and had several false starts in getting it going, I have learned a lot and have really enjoyed it!
I am now at a point where the server and clients are all running normally (success!), but I have a pretty serious mish-mash of half-syncd directories and missing files across multiple computers.

Is there a good way to sync or upload but only in an additive way?
Like if one computer has files X, Y and Z while another computer has files A, B and C; the final sync across all computers will be files A, B, C, X, Y, Z?

Under other circumstances I would just go in and manually sync files/folders, but these are music/media production machines, so there are many dozens of large proceduraly-named files across many dozens of directories nested in complex ways. Any sort of manual sync would not be even remotely practical, but if there was a workaround that was based on directories that could work.
Also if there is any documentation on the general syncing hierarchy/logic (I haven’t been able to find anything straightforward in my searches) I would love to go over it for the future.

Thanks for any help you might have!