Photos you can share using iCloud links, you have to go into the Photos app and share from there instead of adding the photo from Messages, otherwise it looks like it will try to share as MMS (I haven’t tested actually sending it). But that’s similar on Android as well, right? I’ve never used SMS on Android anyway when I could avoid it, always another messenger. One of those other messengers is just built into the iPhone and exclusive to it, for better or worse.
Apparently the EU is trying to make Apple open iMessage up to Android (link). We’ll see what comes of that. More interoperability is always good either way.
I would hope there’s some kind of key signing mechanism to prove it’s the same instance and not just someone else who’s running another on the same domain.