Regarding RCS messaging... It works perfectly on my iPhone 17 Pro Max (main device) and on my iPad Pro which are both signed into the same iCloud account.
Here's my problem: I also have an iPhone Air (with no celluar connection) logged into the same iCloud account, but I am unable to send RCS / text messages from the iPhone Air. I checked my iPhone 17 Pro Max settings, and text message forwarding is turned on for the iPhone Air. Is there something I am missing to enable sending of RCS / text messages from my iPhone Air?