I’ve done everything at point 1 and 2 of your list.
I haven’t deactivated bp-custom.php since I never set up this file and never changed. No idea where it is or if it exists. But I don’t think so since I suppose this is a file you manually set up and I didn’t
Now I am running WP only with BP plugin using BP-default them having reset the permalinks. Now luck!
The problem persist!