This reminds me of how the student meal subsidies are implemented in Slovenia, and in my opinion it was quite unwieldy. You call a phone number and place your phone's earpiece on another device with a microphone. Then, some personal data is transmitted using (ultra?)sound. I remember it being quite unreliable, but that might be down to using the telephone network as the data carrier.
This looks neat, but I don't really understand how it works. I imagine that the DNS record is pointed towards the VPS, and the VPS just forwards all traffic to the actual server via wireguard?
The implementation of the __cos kernel in Musl is actually quite elegant. After reducing the input to the range [-pi/4, pi/4], it just applies the best degree-14 polynomial for approximating the cosine on this interval. It turns out that this suffices for having an error that is less than the machine precision. The coefficients of this polynomial can be computed with the Remez algorithm, but even truncating the Chebyshev expansion is going to yield much better results than any of the methods proposed by the author.