Signal seems to have everything you listed except for the full history transferring to a new device (works on Android but not as well with iOS). Though like you implied, it is done manually. I'm still not convinced that this can be done without making major sacrifices to security because I really do want a trustless system where my messages aren't stored on a company's servers (I am okay with optional backups though, so there's a middleground).

> No concept of selecting a server or anything.

This has been one of the big concepts that has bugged me with Matrix. It also is why I'm confused with why people pit Matrix vs Signal. Honestly I see Signal/WhatsApp/iMessage/WeChat as competitors whereas Matrix/Slack/Teams/IRC is a different ecosystem. But I can't get my parents or grandma to use something like Matrix (or even Slack) but they are able to use things in the former category. In fact, this has been one of the great successes of WhatsApp (looking at India with all the aunties and uncles using WA or China with WeChat).

> Anything missing from this list?

- Pinned messaging

- Other class of extras/plugins: on-device translation, calendar reminders, etc

Pinning messages is important for search, but seems to be overlooked frequently (I use this a lot in slack). I often know something is important and need to find it again in a day or two (e.g. traveling) but will also be talking with the other person and that message gets pushed. Pinning lightens the load of searching. It also lightens the load of backups as most people truly want a very small subset of their messages saved but are only aware of an all or nothing approach.

Plugins will be important as well. To complement pinning calendar reminders are great. Google does stuff like this frequently like when you get an email about a flight and then your phone's home screen will have all the information on it. It's also naive to think that you can think of all the things people would want. That's why smartphones have been so successful, because they provide the ecosystem. This isn't too dissimilar from creating a super app. But there's none where the ecosystem is fully secure.