With extensive work, these tools can all be made to work together under a single username. Every tool needs to be made aware of every other tool, and a lot of the actual federation code needs to be externalised to an all-encompassing server. This can be done without breaking any spec and without becoming incompatible with the rest of the Fediverse, but it’ll require a LOT of work.
You basically summarized my original plan for communick. The basic idea is:
Take the existing services, provide commercial hosting
Use the revenue to fund development of protocol-level integrations, contribute back to open source
Build tools to make it easy for companies to migrate away from Facebook/Twitter/Instagram/WhatsApp and own back their internet presence again
???
Profit / Destroy Big Tech.
Unfortunately, the amount of people willing to pay for commercial providers of social media is embarrassingly small, so I am stuck at step 1 and the best I could do is to build a SSO system for Matrix/Mastodon/XMPP, to let people use the same credentials on all “communick.com” servers.