Privacy

No but XMPP is fine for that. I use both of them. DeltaChat has a way to send larger files in smaller chunks and store them in a persistent cloud drive type UI through the webapps it introduces.

The webapps are really nice and actually work a lot better than all of the Matrix integrations. They may look rustic but I actually use them for almost all of my notes and todo lists now lol. It has a very valuable place in my stack because of how I use it to sync notes and files between devices.

I should be a little more measured in my hyping of DeltaChat lol it's just surprisingly good for how in-development it is already, soon to add location sharing and video chats, they're in testing currently.

Multiple devices is a little quirky as you either need to make a new account (instantaneously) to communicaye with the same contacts and yourself, use the backup (doesn't have a password), or do the transfer thing where you need both devices on the same wifi network.

Again I have tried to stress test Matrix and I did hang onto it for a while to do exactly what you're describing but I just moved on because of the issues I mentioned and losing old messages even while doing everything right. It's some shit with device key storage I think, shit can get fucked up. Maybe a more competent person with a tech job/unemployment would do better than me haha