Cheers, guys - will give Signal a go.
In the few weeks I've had this phone I can't say I've really taken advantage of any of the better text integration with my watch, so it shouldn't be something I miss! Still, why would Samsung release a text app that is restricted in this manner?
It's not really a Samsung limitation, the smart notification api only lets you do so much. You can't, for example, reply to a notification that doesn't exist. On the same basis your watch can reply to a text but not send a new one. Samsung did something priority to enable sending a new message via their messaging app.
It's exactly The same on Android wear. You can't go "ok Google, send text to xyz" and have it work on a non standard messaging app like signal.