You’ll have to brew your own run conditions I think. For me, it’s not a big deal, just a bunch of documents and pictures and not much gets added every day. But termux does have access to network state, and I’m pretty sure syncthing accepts stop and continue execution signals, so a shell script shouldn’t be too difficult. Another possible option is to use termux:tasker.

I said this in another thread, but apparently it’s not widely known: syncthing works fine on termux, there is no need to install any third party code. You do need to run termux-setup-storage to get access to the shared storage that other apps can access, and I found it worth it to set up the termux:boot app to run syncthing on phone boot. This way only uses the official syncthing repo.