For me the ideal replacement would be to have a client supporting both lemmy and matrix. The UI would likely be an abomination tho.
I prefer Matrix communities over Discord servers, but both are mediocre forum replacements. I wish forums would return, but the heydays of PHPBB seem long gone.
Also prefer forums for most stuff, in group chats the relevant info gets lost easily, it's harder to moderate, and impossible to continue a topic in medium-big communities. I think most people like discord over post because they don't like the "create topic each time you wanna talk" dynamic, but then everyone started using thread channels the moment discord added them. It could be more reasonable to have one "general" chat then the rest be posts (kinda like amino i guess?), rather that implementing a whole forum inside an instant messaging app.