> (This may be out-of-date at this point, but I haven't heard of Amazon or Google doing on-device processing for Alexa or Assistant.)

It was out of date 6 years ago.

"This breakthrough enabled us to create a next generation Assistant that processes speech on-device at nearly zero latency, with transcription that happens in real-time, even when you have no network connection." - Google, 2019

https://blog.google/products/assistant/next-generation-googl...