This seems to work pretty well for me. Based on JDR’s comments about the Echo not being usable for anything else while Bluetooth is connected, I never even bothered to try. But after the OP’s comments, I installed the required apps. The voice is not quite as good as Alexa, but it is pretty good. I installed LAN announcer on a Samsung tablet we use in the kitchen, and after installing it, the tablet downloaded some Samsung premium TTS voice. I would say that it’s probably about 90% as good as Alexa.
Also, I have not noticed any functionality loss of the Echo while it’s paired as a Bluetooth speaker. I can still use it to stream it’s own music, and respond to voice commands. I don’t get the notifications while it’s streaming music, but I consider that to be expected.