Interesting goals! Here's my take on nap tracking.
The Samsung Galaxy Watch does a fantastic job at automatically tracking sleep and naps and (automatically) distinguishing between them so you can view them separately. In general, the automatic/passive sleep tracking is very accurate at detecting when you fall asleep and when you wake up, as well as points you (even briefly) wake up during the night, but not very accurate about the sleep stages of 'light', 'REM', 'Deep'.
Here's an example of a day I slept for 5:55, then took a 2:39 nap:
You can click on either sleep block to highlight it and get details, but I can only post 3 pictures per post so I cut one of these.
Then on the weekly charts, you can see the average length of your naps, pretty neat:
Finally, this is how naps appear in the weekly reviews (I only took a nap on Sunday, and had to wake up briefly on Wednesday night):
Also, I have no affiliation whatsoever with any products, just a fan of things that work