So.. I think the android user has to be using Google Messages for it to work. (RCS seems to be fragmented in it’s implementation on Android still so I guess Apple ensure compatibility with the one in use by Google.)
I have 2 androids for work - a Samsung S24 and a Google Pixel.. RCS worked for the Pixel just fine. For the Samsung, even though RCS was enabled on it and working Android to Android, it didn’t work with my iPhone until I installed Google Messages and used that.
Once I set it as the default messaging app, I got a popup saying that RCS was being set up and then complete shortly after that. THEN I started getting read receipts, typing indicators and the tap-back reactions worked properly.
EDIT: You also have to turn MMS OFF on your iPhone so that videos/photos will send in full size as opposed to compressed for text.