Signal is fundamentally centralised. It's not going to become a distributed system like the fediverse, because the protocol's design doesn't work that way. (Also, its maintainers haven't shown any interest in adopting that approach.)

If e2ee email is really what you want, you can already have it with PGP. Various email clients exist that make using PGP possible for a mortal. Good luck getting many of your contacts to use it.

If you also want modern encryption guarantees, like forward secrecy, then consider Matrix instead of email. It already does e2ee and is already decentralised.

Email is already federated and can be end to end encrypted with various methods.

You know there is Matrix, right?

DeltaChat is an E2EE messaging app that runs over email infrastructure. But it's a clever fix more than a real solution.

In theory, as I understand it, the anointed future solution to this whole conundrum is still Matrix.

Sound like some sort of xy-problem here.

There is encrypted email, look into tuta or proton. It's just that "no" (I just don't know any) other service implemented their encryption standard. But it is there. People would just need to use it.

Moreover, matrix is encrypted and federated.