1) I like to minimize the number of APT repositories I use. Debian's and Ubuntu's are well behind on Syncthing versions. Every so often (much more rarely than before), they have breaking changes. As such, my phone can no longer sync with my computers. Perhaps the answer is that Debian should just not be packaging it, because they have such a conservative policy. I'm considering downgrading my phone's version, or building the latest version from source. Worst case I may add their APT repo but I'm not eager.
2) At least with the version I've been using, opting for local-only sync isn't so straightforward. It's per-instance, not per-folder. This means that I have to have my home server running it as local-only, even for small things that I wouldn't mind getting synced over the wire. It means that if I want to have a local-only directory shared between my phone and laptop, it has to go through my home server first. Unless there's a better way I don't know about.
Otherwise Syncthing has been pretty great. I like your no-delete thing, I did not consider that. I could definitely use it for photos.