i use scrcpy

u can view the android ui on your phone. u need to enable debug mode, and then use scrcpy

but for ama messages u can just use kdeconnect or the Google messages web interface

You're shooting down good suggestions here, so I'm not sure if you don't fully understand the problem, or are asking for the moon.

Are you wanting to send SMS from a specific device and SIM, or just ANY kind of SMS?

SMS is controlled from your provider. If you want to send from said provider, you need a device or app that can access their network. T-Mobile has a browser-based interface for this, and I think Mint does as well. Otherwise you need to get an SMS provider with an API to interact with, like Twilio, VoIP.ms, or similar.

My guess would be something from the mobile linux stack, but that might be assuming GSM is used directly, not controlling an external android or something like that which Im guessing youre looking for?

Anyway it seems that for example PostmarketOS uses https://invent.kde.org/plasma-mobile/spacebar Theres prob. more alike. And when everything else fails, its pretty much guaranteed theres some XMPP magic to do that, there always is.