https://community.nodebb.org/post/88242 I think this answers your question. Let me know if it doesn't work.

Ok, looks like I know what I need to know, what's possible, what doesn't make sense, and what could be a workaround. Thank you very much.

Do I understand correctly that I should simply save the collection of X objects like any other data in NodeBB, using its abstract data access mechanisms? Yes, just use the db.setObject and other methods available, this ensures your plugin works on redis/psql/mongodb. Can I expand the user data, topic, posts, etc. with an additional field in the database (probably containing the id of related X objects so that I can immediately ask about the right objects)? Yes, some plugins like QnA do this and add solved/isQuestion fields into the topic objects. NodeBB is very flexible in terms of data structure.

It is fully supported, our tests run on all 3 databases with nodejs 18 and 20. However we suggest using mongodb & redis as that is what we use ourselves and on our hosted clients.

@baris perhaps it shouldn't be .btn-primary as it doesn't often get clicked

It is actually something nodebb specific, when building the active theme nodebb core looks for the overrides.scss file. This file needs to be imported in a specific order for it to work.