I haven’t done anything about this yet, but did check and see that Syncthing 1.19.x is in the OpenWRT 21.x package database at least:

Last time I tried and succeeded at this was a couple years ago, but at that point all I had to do was find a place to download the package for my architecture and install it. It may not still be so easy, but at one point, it was.

The only listed dependency is libc which is interesting. I believe Syncthing is written in Python, so if OpenWRT packages don’t work, maybe there’s a chance it can be made to work another way.