How do I stop my text messages sharing to all my devices

Sign out of Settings/Messages on the devices where you don't want to receive messages. You can't stop just text messages.

That is a change in the latest IOS. What I posted above about signing out should still work. If you are signed out of Messages on the computer, you shouldn't receive any messages.

if what you claim would be true, what is this option for on my iphones with latest OS to this date:

can't find the pattern, sometimes text messages won't forward for a few days then it forwards them all again at once.

This can be considered a security risk because if the mac gets compromised and a MFA request is sent to the phone, access can be granted to accounts only for the specific mac.