Hey there, I've been on the hunt to get a few more and better sensors on my wrist, without having to send them to someone else as well.
As it took me quite a while to realize this, I decided to maybe make this information a bit more discoverable.
The Open Source App Gadgetbridge is a replacement companion App for smartwatches. As it explicitly does not use any Permission for networking, it cannot communicate any of it's data via network.
As there is no official support from any of the larger manufacturers, it's support is somewhat limited, but as I realized now, Amazfit Smartwatches all apparently use the same ZeppOS, and ZeppOS is supported by Gadgetbridge!
There are some limitations, for example DND sync from watch to phone is a work in progress, and SPO2 data isn't synced yet either, but I guess some of the people here might be willing to make that sacrifice.