Thanks for the reply, Nahum. The link you suggested is *kinda* close, but it removes the wrong part and results in a strange plural/non-plural progression of URLs.
Disabling the prefix, I get the following results:
Forum root URL: example.com/forums
Single forum URL: example.com/forum/movies
What I want is:
Forum root URL: example.com/forums
Single forum URL: example.com/forums/movies
What it is currently: example.com/forums/forum/movies
And after doing some searching, I think this is probably NOT an issue with BuddyPress but an issue with bbPress. See this topic from SEVEN years ago https://bbpress.org/forums/topic/how-to-remove-forum-from-url/
Still reading through it, but I’m guessing that I will need to fix this with some rewrite rules, because the author doesn’t see a problem with the redundant stuff in the URL.