Has anyone found a good fitness metrics watch that is more open source (no monthly fee)?

Oura looks new with no monthly subscription fee. It looks like horrible to wear for serious sport though.

I have Whoop but will cancel the monthly fee once my intro period is over.

But Whoop would be worth it for me if instead it would be a flat price own outright + they allow 3p access to all the data not just heart rate.

The whoop in theory is great but it doesn't capture my sport's 'strain' well and the heart rate lags my treadmill band. I can't verify the sleep but it seems to be more accurate at least measuring my interruptions. But if it was flat rate not $30 a month i think it would be good enough.