gotcha on that.
I have a Wahoo Tickr that pairs better with my Hammerhead Karoo on ANT+, and decently, though less stably with my iPhone6,
used to pair with the Karoo occ to cross check the GPS on the Karoo.
Interestingly the Karoo is ~2-5% less on routes, though GPS algorithms easily vary that much and it is also that much less than a drawn course on MapMyRide.
As I was talking myself out of a powermeter, I looked into the various metrics, laboratory quality thru subjective, and feel for me, the subjective RPE exertional rating, speed duration and distance were all I needed, and playing around with the calculated proxy VO2max and "FTP" on various sites were fine for me at this stage.
What I was looking for was a Respiratory Rate Meter that might log that data, as I log the other training mishagas.
It has been fun looking at the correlations between the "gilt standards" of VO2max , watts, and METS , seeing what are reasonable proxies with stable values for use and comparing mine.
I dodged Cardiac rehab, and wanted to see where its (rather fanciful) metrics fit in, as well.
Plus I am ( like many older folks) on beta blockers that further screw up using heart rate as a metric.
RRM are to be had . There is a good paper reviewing their use in monitoring pediatric pneumonia
https://doi.org/10.1164/rccm.201711-2233CI that lists what they do.
I haven't found anything that costs less than a good powermeter , designed for hospital use, as of yet.
I think they may be as good if not better than HRM.
and it would be an interesting comparison to match it with "5 min of huffing" to see if it out performed simple timed intervals with subjective endpoints.
Like the OP , there a a lot of folks who make use of a game of stats to help motivate themselves, and it is always fun to calibrate that with a bit of science.