Resilio and SyncThing both support different modes:
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âmirrorâ mode, where deletes on the source device are reflected on the destination device
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âsend onlyâ mode, where deletes on the source device are ignored.
You want the second option, so if your phone decides to âsmart deleteâ photos that were backed up somewhere else, the backup on your NAS doesnât synchronize those deletions.
If youâre using SyncThing, you want âsend onlyâ mode. If youâre using Resilio Sync, you want âRead Only-Full Syncâ and âStore deleted files in folder archiveâ on the server folder (which should be set by default).
With respect to how I suggest people set up files, itâs up to you: I personally like to have my photos in one directory hierarchy, so I use automatic organization, but Iâve retained all my Resilio archives separately, as well (disk is cheap).
If youâd rather maintain a single copy of your photos and videos, you can either opt out of automatic library organization, or you can periodically archive your Resilio backup directory (take care to not delete the files that the app uses to maintain sync state, and realize you shouldnât archive files on dest that still exist on src, as the sync app will re-transfer everything you just archived).