I don't know Dicio, but I mean can you just leave the app open? Because that's essentially what the other assistants are, just devices with the app always open.

If you can leave the app open, and it otherwise complies with your requirements, then we already have a FOSS voice assistant, it just doesn't have its own dedicated hardware yet. But if you would dedicate some hardware to it, like an old phone, then it could be largely equivalent.