Most of the stuff is notetaking, people will use markdown based tools and sync them. That's what I do with Obsidian, but it seems like pretty much every flat file app on Android has a lot of SyncThing users.
I haven't looked into the internals of ST all that deeply, partly because IIRC it's written in Go(Like so many P2P apps) and I'm not familiar with it.
From what I know of it, you could probably port it, just because the ESP32 is really powerful, but it would be a big rewrite. IIRC ST uses some kind of embedded database, not SQLite but similar-ish, I think it might be NoSQL.
I looked into it at one point because I wanted to add some features, but abandoned that when I found out how committed the devs are to sticking to the core functionality only.
I think if I were going to push it to ESP32(Which seems like an amazing idea) I might look into a custom embedded-first protocol.
It would be amazing if SyncThing could add support for a lite version of the protocol, but I doubt they would. The community seems to not mind forks though, maybe someone can make a "SyncThing for IoT" fork?