Echo does not currently support text to speech. Nothing to do with smart things, Amazon just hasn’t added that. A lot of people would like to see it, so put in a feature request through your Amazon app.
Echo can act as a Bluetooth speaker, which means it can be done, as you noted, with a workaround but you have to have another device.
The following thread shows One way to do it. You’ll see several posts from me which are skeptical, basically asking “really?” But, yeah, really. They found a way to make it work. But it’s not using the text-to-speech feature, which is why it doesn’t show up in the base rules you’ve been looking at. But again, not a SmartThings issue – – it’s Amazon which hasn’t yet supported text to speech. As soon as they do add that feature to echo, it should be usable in SmartThings.