iMessage integration won't happen because Apple has closed down their API. There have been reverse engineering efforts but such a system is never reliable enough for comfortable desktop integration imo.
I've never come across iMessage lock-in as everyone I know either uses WhatsApp, Telegram or plain old SMS so I don't know what limitations you might run into in your setup.
Come to think of it, KDE connect on iOS won't happen (GPL conflict with app store, restrictions to apps, limitations on background execution). I suppose I shouldn't have expected anything else from the Apple ecosystem.
I can also make and receive calls from my Mac, provided the Mac and iPhone are on the same network.
Thanks for the clarification.