@batfastard
First, Google Assistant is free also. I don’t know why you are thinking so.
Second, as your description, I think you are running my old MMM-Assistant. It was deprecated and I have no plan to maintain. Sorry.
I looked inside MMM-Selfie mentioned. Very interesting. It has no other interface except my MMM-Assistant and MMM-TelegramBot. On one hand, thanks about it as creator of both module, but the other hand, that module was too dependent to other modules. Not expandable.
The best simple solution I think is;
Modify MMM-Selfie to receive trigger from other modules beside MMM-Assistant and MMM-TelegramBot. Seems not so difficult. just add .notificationReceived() for some notifications from other module.
Then, Just use MMM-Hotword(you are already use it?) to emit that notification. You can make your private voice model, or use just universal model for everyone.