Isn't the just based on a matrix server connecting to all your accounts?
What the EU forces them to do is to be able to send messages across service borders, so you could communicate with someone on WhatsApp, for example, without having an account there yourself.
I do share the most upvoted comment's skepticism though - Meta and Apple will fight this tooth and nails and make it so cumbersome (and opt-in, of course), it will have no relevance in practice.
As a Signal user I'm also not happy that the very least I have to share with Meta is my phone number (that's also criticism towards Signal though, I guess).
Of course not. At the very least you have to be fed up with Windows before moving elsewhere. If you want Windows, stay with Windows.
You shouldn't continue using Windows 10 after end of life though. Once it doesn't get security patches anymore, it is a time bomb. And since the code base is easily 80-90% the same across versions, new vulnerabilities patched on newer versions are just hints for malware devs making the obsolete version even more likely to be attacked.