Rhasspy and Mycroft are pretty much your only options here, but prepare to spend a lot of time configuring them.

Unless you've really got something to hide, I'd just accept that Amazon/Google already know everything about you based on your phone, search history, and shopping habits, so sending specific phrases after a known wake word probably isn't the invasion of privacy you think it might be.

Alexa is better than Google assistant in my experience (Alexa in the house, Google in the car), and the Nabu Casa subscription is well worth the cash.

I know this isn't the answer you wanted, and I'm totally prepared for the torrent of downvotes and people telling me how wrong I am, but hey, this is the internet, put 100 of us in the room and you'd get 120 different opinions...