wouldn't it be enough to propose RFC for secure email handling by email clients?
No. The problems with encrypted email are unsolvable by RFCs.
Creating completely new protocol might hit problems solved already in existing protocols.
This is why https://github.com/soatok/awoo-specification is still an empty repo: It's work I planned to do after I solved key transparency for Fedi and E2EE happens.
For example user friendly, decentralized key distribution has no great solution, if it should be secure at the same time?
Literally, https://soatok.blog/2025/12/15/announcing-key-transparency-fediverse/