In the end I went with django for one reason: The admin.
Why there's nothing comparable in the java world I don't know, but for an early project it's invaluable to be able to focus on your app while having a good enough admin.
Almost everything else favoured Java tbh (cheaper devs - unlikely for kotlin though, static typing, performance, etc).
Having said that, their REST thing was garbage. I seem to recall it didn't play well with the DB repos or something. Django ninja is much better from what I've used so far.
Oh yeah, also I recall you're right about the lack of user registration stuff. I read most people use keycloak or something, but that's still more stuff to run and wire up than django allauth.
Also I'd question why you want to write the app in a different language. It'll double your work for little gain unless you have real need. Just chuck a Web shell around a responsive website and fill the gaps for the native stuff.