Yea, that’s a challenging part for sure, one that I still deal with.

It can be done, but you need to configure the sync jobs “just so”. Send only from the phone, don’t sync deletions, etc.

In the past I’ve setup a master folder on my phone and used subfolders for “upload only” Syncthing shares. Then wherever I put a file on my phone it gets synced to the appropriate folder on my server.