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.
I do wish there was a smaller LongCat model available. My current AI node has a hard 16GB VRAM limit (yay AMD UMA limitations), so 27B can't really fit. An 8B dynamically loaded model would fit, and run much better.