As Snowden told us, video and audio recording capabilities of your devices are NSA spying vectors. OSS/Linux is a safeguard against such capabilities. The massive datacenter investments in US will be used to classify us all into a patriotic (for Israel)/Oligarchist social credit score, and every mega tech company can increase profits through NSA cooperation, and are legally obligated to cooperate with all government orders.
Speech to text and speech automation are useful tech, though always listening state sponsored terrorists is a non-NSA targeted path for sweeping future social credit classifications of your past life.
Some small LLMs that can be used for speech to text: https://modal.com/blog/open-source-stt
The HA Voice Preview is a pretty solid device, but you’re right, there isn’t really any ready made Echo/Google Home Mini replacement device - primarily because all those devices are generally sold at a loss, or at cost at best, and subsidised by your data being sold.
You won’t be able to make a Google Home Mini contender for below $50, and at that price most people will opt for the former. Good quality speakers, microphones, local processing (like the XMOS chip in the Voice Preview) all cost money, and there’s no subsidy to be made. Some older Echo devices are rootable, but the hardware tends to be somewhat exotic (meaning no open source support for specialised components), and there’s little ongoing third party support (focus has been on the display-equipped models, and to run Android on them).
All in all, “cheap” and “fully local open source voice assistant” don’t really coexist.