Switching sync apps after a year-long gap, how do I avoid re-uploading photos already on the server?

I used to sync my phone's camera folder to a Linux box with Syncthing, but I stopped about a year ago. So everything older is already sitting on the server, and roughly a year of newer stuff exists only on the phone. My question is basically this: if I switch to a different sync app now, will it see what's already there and just send the missing year? Or does it start over and copy everything again? I don't want a second copy of the old photos on the server, and I really don't want to spend weekends afterwards hunting down duplicates. It also needs to work for family, so I'm after an app with a one-time setup that points at my services. No scripts, no terminal. Files need to land on disk as normal files in a readable folder tree, not chunked into a blob store (looking at you, Seafile). I run Immich with the archive as an external library, so the folder layout is mine to control, which is why Immich's own backup is out. As for why I'm dropping Syncthing: the Android app got archived, but the real reason I'm leaving is that it crawled once the two ends were on different continents. I travel, so that's not a one-off, and I do run NetBird so reachability wasn't the issue. Not looking to debug it, just to replace it. Please share any suggestions that you have for this. TIA!