I must express myself quite poorly. It is not a point about technical knowledge, in fact if you were to know more about the topic than I do, I would expect you to even more be upheld to higher standards and thus not promote a bad solution, even more so assume it's the only one. I can't imagine that even a PhD student who is supposedly at the frontier of knowledge in their very narrow field would assume no alternative is possible, or will ever be. This even more the case without having both a complete understand of the landscape but also about OP's actual needs, which is probably hard to express clearly and thus leading to a lot of assumption. Here maybe a simple loud alarm from a BT speaker going out of range might be enough.
My whole point is that abandoning hope, and leading others to do so, is worst than actively finding for a barely OK compromise.
Anyway I don't want to invest more energy on this discussion unfortunately so simply wishing you the best, thanks for the clarifications.
It's a tricky situation to navigate.
There is the technical aspect, namely is it actually feasible, but itself wrapped within an economical and political context, as I've highlighted in another thread on this post.
On one hand we learn from Snowden's leaks about an entire surveillance apparatus, we might also have a conceptual understand of limitations via articles like "On trusting trust", plain incompetence and shortcuts for large companies, so all that and more invite us to be very prudent. Those are actual justifications for questioning what hardware, if any, can be trusted.
Yet... one can't go from those justifications to speculate. Yes there might be flaws, intentional or not, in both the design or the production or both of chips. Still, it's not because it's conceptually possible, or even that it happened before, that it does happen today and at scale.
Your System76 is an interesting example and it's a bit like my Banana Pi tinkering, or even more limited (yet exciting IMHO) the Precursor. Namely it's a very costly trade off today to "work" with hardware one can (at least try to) understand better, hopefully itself leading to better privacy and security. In the end most of us believe the trade off for more affordable performances trumps that deeper understanding.