Yes, their web & mobile client https://riot.im is very polished now. They are also rolling e2e crypto.

It may be a little over engineered for what I want it for, but it does work! I started up the reference server in a docker container and wrote a quick bot using the provided python-sdk and can interact with it using the client on my phone.

The main downside(from my perspective) is that it's a lot of moving parts, and I think I'd be more happy with just irc over tls since I at least know how that works :-)

It's mostly a question of mobile support: IRC (over TLS or otherwise) requires a continuous connection, where matrix and signal are designed to support polling a push notification endpoint.

Out of curiosity: what makes you say Matrix is over-engineered?

Oh.. maybe not over-engineered, but just more involved than what I need. I mostly just want a way to move messages back in forth. I don't need voice or video calls or anything like that.

One of the use cases Matrix was designed for is Internet of Things, so I think it's just fine for you.

Video calls come thanks to WebRTC in browsers, Matrix is just a transport protocol. I think it's quite light.