it doesn't solve all of them but a few at least. i don't believe using pgp itself is a problem. if it was deltachat could replace it with another better encryption.

metadata is a problem. that could be solved by removing messages from the email server and storing them elsewhere. (or storing them back on the server with the metadata removed). that doesn't prevent intermediate mail forwarders from keeping a copy though. i think that is the real problem. deltachat doesn't control the communication channel and can't prevent leaking. i don't know how matrix or jabber compare here. if they use intermediate servers to forward messages the problem would remain.

message content leaking is not a problem because messages won't ever be decrypted on the server.

long term secret leaking is a problem tied to the current implementation of pgp. deltachat could change that (and maybe it does?)