Regarding recommendations, I think we're very much in a transitional period. Right now, with the bill that's being looked at regarding encryption in the UK, I think it's a wait and see in terms of what precedent that will set for other countries as well.

Regardless of the border BS, SimpleX is an option though the lack of interoperability as a transition-ramp could be an issue, but personally I'd be more inclined towards Matrix as I've used it extensively in the past.

However, I think even those may full prey to issues that Mastodon is known for, not to mention legacy border issues.

Instead, I'm more interested in seeing Nostr as an option, for instance #0xchat is a good one but not as featureful yet.

Like I said, wait and see. As for right now, can't go wrong with #Matrix.

Lastly, I don't think it's just about being able for YOU to verify the code, but for it to be openly available and auditable. We can't expect everyone to verify source code throughout their entire life, but there are always those willing to do so, and a million eyes on pieces of code is better than no eyes at all - That's the base proposition with no compromises that I think there are no opposing arguments for, in this day and age.