english is not my native tongue and technical terms are especially challenging - so please forgive me, but I want to toss in some ideas and maybe you can make sth out of it.
If you believe that the amount of oxygen is essential, than please keep in mind how the lung works. the comparison to a plastic bag where all that matters is the volume is a bit far off.
first of all the pressure: a higher pressure increases the "dead volume" which is no longer available for breathing. (it's the amount of air that gets pushed into your lungs by the higher pressure and cannot be exhaled)
higher pressure (which is only in your airways and not outside your body) results in an increased muscle effort. so you have to try harder to breath against that pressure. (try coughing with the mask on^^)
the second thing is the amount of air that your lungs can work with. if you breath in short and deep the amount of "fresh" air is higher but it doesnt get evenly distributed thorugh the lungs. If you breath slower (and shallower) the distribution increases but the amount of fresh air gets reduced.
I myself found it really challenging to breath during the first weeks. My machine has a function (might be the same as the ramp) where it start with the lowest pressure for a given time and slowly increases. Really helped with falling asleep. I'm running on APAP - not CPAP. During the first weeks I reguarly woke up after 4-5 hours due to the pressure and the increased effort to breath. (I'm OSA so the good thing was: I was than really well rested (compared to before) and simply got up)
But the problems with the breathing effort on higher pressures totally vanished after that. (I believe I have build more muscles
)
I'm also not so sure if the "flex" setting works if you are using the CPAP-mode (C for constant pressure). If so it would be the same as some sort of bilevel-PAP. Maybe you try the APAP mode? (and with a decreasing flex-setting to get used to the "new", higher breathing effort)
But maybe I'm totally mistaken ... I'm only a few months on my mask and still have to learn a ton