I currently configure signal to back up into a directory that's synced to my server via syncthing
Which is what I used to do and the problems were Signal keeps two copies of the file by default and puts them on your device memory. So your device ends up having 3 copies of the same data, on the same device.
This is rather impractical if you have a lot of content in Signal, which I do because my family use it extensively to send photos and videos (which is the sort of thing you generally really want to backup).
I really want the backups to stream off to my own version of the backup service, and do the neat deferred media thing which would be great for keeping long range archives (i.e. by letting them get picked up by my server's regular snapshot to backblaze).