Health tracking devices with selfhostable backend

Are there any good ones? I currently have an Apple Watch and sync it to OpenWearables, but I'd much rather not have my health data in any cloud. Is there any hardware for that?

I don't care about the type of hardware. It can be a ring, watch, band, etc.

Needs to be compatible with iOS.

Health metrics I care about:

HRV
VO2 max
HF
Sleep + phases

Not must-haves, but nice to have:

Skin temperature
Breathing frequency

https://redd.it/1x3al7f
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