First of all, I'm convinced that it is only those mask users who truly cannot ventilate through the nose who must mouthbreathe. It can be stopped by other mouth breathers; we have to adapt to whatever means are necessary. My successful measures are too onerous for many persons to accept unless they are desperate. Breaking away from FFM was not that hard. Every improvement I made took at most a few nights to adapt. Stents, a bovine valve and a pacemaker were persuasive. I attach a pic showing (hidden, covered) use of (click) Gel-e-Roll-seal (by Silipos) and its elastic DIY retainer as well as a vest mounted supinity block (a vest-knapsack holding two nested boxes, which I only lately found was not necessary, the benefits of a--now--4" cervical collar). I needed firm pressure against the clinging (but not sticking) seal so I used the elastic top from a worn out compression stocking and velcro to make a retainer. Others may get by with less, like a (click) Scunci
For too long I believed I had to use a FFM because of mouthbreathing and a mouth that gaped open supine and with lips that fluttered in CPAP sleep. At least with a FFM mask, that driest--skinned lizard that used to crawl into my mouth could not get in with my mask mounted. I tried a whole variety of chin straps, tapes, etc., but was driven by sleepless frustration with leaks and noises of several FFMs and hybrids to find a way to use nasal pillows, the P-10 for a few years now--with a cervical collar I was slow to adopt.
Now, LL is rare and L is mostly well below 10 L/min--no more awakenings to mask noises and loss of therapy. The P-10 relies on both minimal skin to pillows contact and, with the exception of the "bleep", I believe it has least need to bridge over contour variations of nose and face. This sidesleeper has no complaint; a large head spares me those strap loosening problems which, most crudely, even one scissors snip and a staple will fix if shrinkage after washing does not.
Of my 3 once-needed, helpful, and adjunctive devices I have listed, only the accelerometer remains operative (but now idle). My second CMS50I died, too, of old age and the so-so Dreem 2 needs head-positioning band repair--if, indeed, Dreem even supports use of it now.