Correcting some misconceptions…
Element for Android doesn’t support searching in encrypted channels
That’s true of regular Element for Android, but it’s being replaced with Element X (which is built with Rust). I would expect search to be added there if it isn’t already.
and I think you can’t use E2EE in the browser at all(?)
I have done it in Firefox, so that’s false. Perhaps you had trouble with a specific browser?
plus basically every other client has even more drawbacks when it comes to E2EE.
Nheko handles E2EE just fine, so that would seem to be false as well.
Since you’re looking for recommendations, it would help if you said which clients you tried and what problems you had with them.
In case you haven’t seen it, you can set a Features: E2EE filter on this list:
https://matrix.org/ecosystem/clients/
A web forum is far better in most cases. If you can’t manage to run your own, there are plenty of lemmy servers that will do it for you. Even an email list (with searchable archives) would be better than Discord.
If you have collaborative documents that outgrow the forum format, use a wiki.
If real-time chat is needed, irc or matrix.
A project hosting its community on Discord is a project that won’t get my contributions.