The good thing here is that you don't need to trust the server in order to have a secure communication since your clients decrypt and encrypt and not the server.

Yes they can optimize with things like this but that doesn't make it insecure. It's still the most secure solution that the average person can use.

Threema doesn't even have the server open sourced at all, are for profit and their encryption has been compromised.

Session is shady.

Matrix is a metadata nightmare due to it's federated aspects.

SimpleX is the only thing that is secure, anonymous and good in this regards but it has some small details left that prevents people from switching. I.e. simple things like the fact that you can't see an overview of your images and videos sent in a chat without scrolling up all those messages. It seems trivial but for the average user stuff like that is important since they know it and use it every day in other messengers.