Omemo >8.x has some encapsulation of metadata. I am not aware of an easy article about it though. Arguebly this is adding something similar to what Signal partially achives with work-arounds only possible due to their centralized infrastructure and single app only architecture, but doing that in the encryption layer is ultimatly the better approach.

Cross-signing of keys ultimatly outsources trust to someone else. From a security perspective having to manually verify each key yourself is the best way. Cross-signing is a bit controversial in xmpp developer circles as a result, because many think it adds a lot of internal complexity while being only marginally more secure than TOFU.

I don’t think I can give you any recommendations for good servers, as it is very context specific. Jurisdiction is IMHO among the most important issues, because even if your encryption is water-tight it doesn’t help you much if the provider can be coerced into tracking you or can be easily shut down and replaced with a honeypot.

As for calls: the popular xmpp mobile apps do have well encrypted peer to peer calls. Currently most are limited to 1:1 calls, but some others are experimenting with small group calls as well (currently only Dino, Movim and Libervia support these).