As far as I know all the Nest and Echo conversions are quite involved. You need a specific, mostly old version of the device, custom circuit boards... I don't think I'd try it unless I happen to have that exact version in my old electronics drawer or otherwise know what I'm doing...

The voice stuff from Nabu Casa should be alright. I'm not sure if they still label it as experimental... You might want to try with your smartphone companion app first (before ordering hardware). That comes with the same voice assistant and you can see if you like it. It requires some compute power on some server, though. Either a monthly subscription with NabuCasa, or you install the addons for speech to text, text to speech, etc on your own server.