I personally use Matrix and bridge all my social accounts to there. It works super well, because my friends don't have to switch their app, but I see everyone in just one app. matrix.org/bridges
https://element.io/element-one -- I'm using a Matrix Signal bridge (meh, it bugs quite often, kicks me out, doesn't properly update keys when people get a new phone). Also does not support calls etc.
This is what I do too. Sadly, there's no good call support which prevents me from uninstalling the relevant apps entirely, but all messengers pipe through Matrix now.
I'd much rather see people move to Matrix (or Signal, I suppose) than to keep having to use WhatsApp but that's a pointless fight I will never win.
Essentially a Matrix-powered service focused on bringing a ton of inboxes. I've heard it's great, but never tried it myself.