It’s been a few years since I’ve been down this path, but last time I went exploring one of the main challenges was getting decent hardware. The microphone array on something like an Echo at the time was far better than anything I seemed to be able to achieve without buying in to the Amazon or Google ecosystems.
The asthetics are hard, too. If it can't be completely hidden and has to sit out in the open, it needs to look decent (aka "spouse acceptance factor").
Amazon and Google have both done pretty good here, and I hope there's a good option to replace them in the future. Maybe it's possible to transplant an esp32 into one of those devices? Maybe sometime will start selling something.
The same as back then: You can get pretty damn close with a used PlayStation Eye webcam (I have 5 lying around for that reason). Unless something changed very recently, the next steps up cost more than a whole Alexa device (e.g. ReSpeaker Core).
Is there better consumer stuff available now?