You’ve done your homework and seem to have covered all the possibilities, congrats.
I’d have suggested an Echo or Nest with the microphone cables snipped off (or permanently muted) if you’re brave enough to open it up. However the local audio part would definitely be a deal breaker for you.
I haven’t dived down too much, but maybe the Lenovo ThinkSmart view might fit your needs?
Hopefully it’s still available pretty cheaply on the second-hand market, and there’s a large thread right here detailing jailbreaks and installing custom ROMs.