The flaw in your reasoning, I think, is that 'sleep debt is only increased when awake', which is... Not true. Sleep debt is absolutely accrued when sleeping - if the sleep is not REM/deep sleep, or is very low quality, or is at the wrong time of day (regardless of light quantity though more light does make it even worse).
Circadian rhythms exist despite when the body may initially give signs of sleepiness, and, per the details of a woman that recently left a 500 day zero-daylight experiment living in a cave, exist regardless of any direct daylight. Oversleep, therefore, is always a constant threat, regardless of how much the body may tell you it wants to sleep - the body may want to rest, but the brain may not be primed to do all the necessary tasks it does when actually sleeping, so, not only is not all sleep good sleep, but a lot of sleep your body tries to convince you of, can actually be bad sleep!
Sleep quality, not just quantity, is a known factor in total rest. You might sleep for 14hrs, but might only get one full cycle of good quality sequential undisturbed REM sleep, and the rest might be fitful tossing and turning and gaining little actual value from the sleep, your brain effectively in a not-sleep state, and thus not gaining 'rest'.
Oversleep is a real thing. As above, sleep quality is a thing, and can drop if you persist in sleep beyond what is required by the brain - which, unfortunately, is NOT always an accurate indicator of when it's good to sleep and when it isn't.
Daily consumption, daylight, exercise (intended or not), anxiety, light tricking your brain into the wrong mode, literally anything that can drastically alter the chemicals pumping through your body can bring on, or remove, sleepiness in surprising speed. Sleeping at these times isn't necessarily going to be good true REM sleep, because your brain isn't actually ready to sleep, it's just getting some of the chemicals that tell it now MIGHT be alright for a snooze.
Circadian rhythms are more like 'processes our brains evolved to know when it's time to try to properly sleep', and unless you have a circadian rhythm disorder (which is often considered distinct from fatigue or sleep quality disorders), sleeping before your brain has gone through those processes (daylight dwindling, daily activity decreasing, been awake for an extended time, among others) will lower sleep quality. Much as it sucks to acknowledge, a lot of the generic advice given for better sleep isn't meant to be 'advice to strive toward', it's more 'this is the basic framework of how we've found it to work over the last several hundred years - deviate either at your own self-knowledge or peril'.
Typically, best results for naps are less than 40mins (not enough time to shift from phase 1 of sleep, thus not waking up feeling like groggy death) - this is for a quick body-rest, to shake any false feelings of sleepiness. Then, somewhere between the 2.5 and 3.5hr mark (give or take) - long enough for roughly one good REM cycle. A mini-night's sleep, effectively.
The HARDEST part, IMO, is correctly timing it (and that's saying something). It's less than 40mins from when you actually fall asleep. Not from when you lay down. Same with the longer nap. Same with it all.
'8hrs' recommended sleep a night is an average that accounts for 'time going to bed and trying to fall asleep', as well as daily and individual differences in circadian rhythms. 6-9hrs is actual sleep time, 8-11hrs is more realistically the 'winding down for bed' sleep timeframe for most people, which is yet another way Capitalism sucks the life out of the worker, since daylight is time to spend and time is money.
Experience: a carer of 10 years to my partner, who has fibromyalgia and an anxiety disorder, who often oversleeps and rarely ever benefits from it. ~10 years of observations of her sleep cycles, experimenting with different wake-up times after naps, how she fares after exercise, etc, sadly tends to align with a lot of the inane advice around 'screen time before bed', as well as the above assessment of nap times and REM cycles. Despite being a 12+hr sleeper undisturbed (which causes her to be exceedingly tired most days), when I get her up and active after ~9hrs of actual sleep, she might take a nap later in the day - less likely if she is also physically active during that day.
So, counter-intuitive as it may sound, try cutting back to regular sleep time, and focusing on sleep quality instead - lights, distractions, lack of exercise, high energy/caffiene/"heavy" foods before bed, all things that need to be ironed out before any other cause can easily be assessed.
Sleep 'debt' is not something we can consciously and easily 'pay-off' without ruining our immediate sleep schedule, but where available, the above timings of naps have been shown to help in the course of a standard day with mitigating sleepiness and ensuring total amounts of quality sleep per day.