To clarify all Joomla 4 tables need to be utf8mb4_unicode_ci collation?
Ideally they should be, yes.
Regarding the "stdClass as array: error, there was a recent post about the issue. Seems to need adding back code for their CB integration as indicated here:
I don't see what that would have to do with CB. It's a regression in Kunena as they've noted.
Sorry, I don't have anything further to suggest. You'll need to work with Kunena on tracking down and fixing this.