The echo can definitely be used as a Bluetooth speaker. The issue is that it doesn’t support TTS ("text to speech), all it can do is act as a realtime streaming speaker for another device. So you have to set up some other device to decide what sound file to send to the echo and when to send it.
You can also just use any cheap android device for voice notifications, Among other things, that’s how LANnouncer works.
For other devices see the following list. It has both very cheap and very expensive speakers.
And this for just the cheap ones, but it hasn’t been kept as up-to-date as the master topic.