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mmobder please just understand this fundamental principle:
If you run something, no matter how invasive. And you cut off all it's ways to communicate with the internet. And you only connect trusted devices, like a GrapheneOS phone, to it, where you know that those will also not phone home any data. Then that thing it private.
I do the same with a Xiaomi Mi Band 8 and Gadgetbridge, it works great and is fully, 100% private (after the initial setup, with no personal data, over a VPN)