I have the same watch as you and although it would be great to be able to use the watch as a sensor. I don't think it's worth the effort namely because:

1) Withings is still pretty niche. Not that many people own a Withings watch.

2) The people who use a Withings watch are generally not the same people who ware for the most sports focused watch. Those customers will either buy a Garmin watch or increasingly also Apple Watch or Wear OS watch (Samsung/Google).

3) I used the Withings watch side by side with a Pixel Watch 2 and Polar H10 HR monitor. While the Pixel Watch is impressively accurate compared to the polar, the withings is not on the same level. It's not terrible as long as you stay still (which would be the case for breathing exercises) but not comparable to the pixel watch.