group encryption could be used to support Encryption for Shared page

https://blog.trailofbits.com/2019/08/06/better-encrypted-group-chat/

https://messaginglayersecurity.rocks/

Group encryption is a nontrivial thing currently being developed. MLS is one attempt to solve the problem and make it scale. We can borrow their approach.

MLS was recently (around 2023) made a (proposed) open standard: https://datatracker.ietf.org/doc/html/rfc9420

So to illustrate — this is cutting-edge stuff. In theory means we can centrally relay texts with great security properties without taking on a massive performance hit.