So reading your requirements I’d say all you’d need to do is set the data directory to NC to the syncthing directory. When new files are added SyncThing will see them and replicated, similarly I believe NC runs a scan periodically to pick up files in the data dir (occ files:scan) so it could well be possible to replicate to the NC Pi too.
MariaDB and PHP7 are fine, NGINX isn’t officially supported but there are several topics about it on the forums here with example configs and such.
I only envisage permissions being a concern, since NC requires the web-user (www-data, etc) to own the files. That’s probably workable though with some thought.