the EU is definitely focusing on Meta, but iMessage, Signal and RCS (Google) are large enough to all be in scope of the regulation.

Signal and RCS are not in scope. You can see the targeted services here, for communication it’s currently just WhatsApp and Facebook Messenger. iMessage is being discussed but that’s it. Signal and RCS don’t have a large enough share, Signal is tiny and RCS is not even a blip on the radar. If anything it’s going to be Viber and Telegram next. Viber has a 25x larger userbase than Signal, Telegram about 10x, and WhatsApp is about 75x. Here’s a chart to give you some idea.