dconf watch / command is (hopefully) reliable… for what it does, So watching changes written in the database managed by the
dconf daemon. The changes done on schema using another backend (like, things written in a keyfile) will not appear here (that’s an unusual case, but it happens, I think GNOME Web uses that trick notably for things related to web applications).
You probably know about it, but
gsettings list-schemas will allow you to… list schemas. So you can probably start
monitoring every of it that way. But I don’t know much about the
monitor function, so I won’t help you here (regarding its behaviour with relocatable schemas, notably).
By the way, why do you want to do that? I can help a bit if you want to write a program doing that, using the GSettings API (maintainer of Dconf Editor here).