Is my go to "just give it a try!" recommendation. Its a smaller/Japanese sized one and its darn cheap. Honestly its probably my favorite having tried out a few more expensive ones. These ones aren't adjustable and honestly they don't really last me more than a few years as the hulls kinda squash down eventually but they are relatively inexpensive so I'm alright with it, the old ones are still useful as couch/travel pillows but less perfect for sleeping.
I've also tried them that had foam pieces inside too but I was pretty disappointed with the two of those I've tried. Many of the more expensive (like $40+) ones allow you to adjust filling and replace if/when it deflates.
No sinking feeling at all, you can kind of 'pile it up' so you get more cushion or compact it for more firm support.
good luck going forward --plenty of good advice ITT.
One "accessory " that helps me with sleeping with my full face mask is a Buckwheat hull Pillow . it can be formed to help make room for the mask while supporting your head.
I could not get used to a nose only mask. the chin strap was a deal killer for me.
there are also other kinds of CPAP specific pillows out there.