That's a good suggestion to use the Garmin as a reference. I don't have the "compatible chest strap" yet. I'm going to hunt for a 2nd hand Polar H10, as it seems it's going to provide the needed data, but if someone has other suggestions, I'm all ears.
Most Garmin watches can be plugged with a cable (usb-c or else) and then accessed like a mounted drive. I grabbed .fit files from the Fenix5 and also from a forerunner 235, really easy, no cloud needed, no softwares to install.
And one more thought: don't hold yourself back if you not have a chest belt yet.
There is a good chance the data is so clear with bangle close to foot that no reference is needed anyway.
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@Mi