Welcome @eckjr, I also have a full face mask and try to breathe through my nose when I'm conscious. I used to focus on the mask seal but like you I still get lower AHI numbers with not so good of a mask seal. I do try to sleep on my sides as much as my aching hips and arms allow. I notice my AHI is a little higher on nights where I've spent more time sleeping on my back which I'm thinking correlates with more mouth breathing. I'm thinking about getting a nasal mask and using a chin strap to see if that helps some. I'm used to my full face mask but if I tighten it too much I get some pain on the bridge of my nose.

I used to have a lot of problems with nasal breathing which was due to a clogged nostril most of the time. About a year ago I was doing a morning/evening nasal rinse which helped keep my nostrils open and made it easier to breath, then six months ago I switched to using a SinusSonic device which has helped me with my breathing and keeps the nostrils open as good as if not better than using a nasal rinse. Plus I like using it a lot better than doing the nasal rinses. Here's some information on the topic:
-- A novel device combining acoustic vibration with oscillating expiratory pressure for the treatment of nasal congestion:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7317541/.
I find that my sleep medicine PA or doctor are very helpful when it comes to questions I've had. Are you able to discuss the question with your doctor?