Syncthing is in dire need of an iOS client. There's an unofficial one, but it was unmaintained and didn't sync with recent versions of Syncthing last I checked.
I feel like there must be some iOS limitations as well. Dropbox can't even sync files in the background, so if I take a bunch of photos, I have to leave it running and touch the screen every couple of minutes in order to get all the photos uploaded. I feel like Dropbox apps are generally pretty good, and this was way janky, and had to be an iOS limitation, but I'm not 100% sure.
iOS apps (including Dropbox) can sync data in the background, but it's a bit janky. You have to give the app permission to access location services. It'll upload a bit of data in the background every time your phone location changes.