Thanks for the clarification.
Congrats on your round. Best wishes for turning that into a robust truly decentralized E2EE communications platform that the average person won't shy from. Btw, I hope SNEK sticks!
Thanks :) Also, another improvement to protect metadata that we're working on is https://github.com/matrix-org/matrix-doc/blob/rav/proposal/r... (and relatedly https://github.com/matrix-org/matrix-doc/blob/neilalexander/...). By giving each sender in a given room its own public key, you can de-correlate the identities between rooms by blowing away the mapping that links a matrix ID to an identity in a given room. (Of course, a malicious actor could deliberately monitor this and de-de-correlate - to which P2P is the longer term solution.)
Oh yes, great. I was wondering yesterday if you'd looked at an approach like this, but hadn't gotten to looking at docs/code. Thanks for the pointers! It's definitely a good start for obfuscation.
All the stuff you enumerate is metadata that your server stores in order to store your conversations. Until P2P Matrix lands, if you're worried about metadata, only talk to people on servers whose admins you trust.