Hmmm. Mebbe it's the mask. Side-sleeping is going to be tricky. Back-sleeping is bad. Whaddayagonnado?!?!?
I use the same mask and have no measured or noteworthy rates of leakage over the course of my 20 months of therapy. But I do leak. Some of the night. It's just that it is small.
When I get leaks, I usually awaken. This is because the leaks happen upwards toward my eyes, and the eyelashes fluttering give it away. I'm instantly awake. But, because I leak so little, and not all night long, when it happens significantly it wakes me up. Are you being aroused similarly? If so, you should be quite cranky by morning, and desperate for a nap by the end of anything passing for a lunch break. Or sooner.
When I first started out, I was convinced I'd have to get used to sleeping on my back because of leaks. In fact, to this day I mostly fall asleep that way. However, about month 6 or so I realized I was turning over onto my side, something I have always done, and was blissfully asleep in no time at all. I routinely turn from side to side, and I have yet to get a red angry face next day.
So, it can be done. The fit, and learning how to pound and shape your feather pillow, are key.
You say your headgear is good 'n tight, but still there are no marks next day. Great! Maybe you just need to refine the way your face, your cheeks and mask, interact with your crooked arm, your curled wrists if you keep them tucked up there as I do, or the pillow.
Also, you mentioned tossing and turning; I'm afraid that is not going to do you much good for a couple of reasons. If it slows or prevents the onset of sleep, that's bad. If it tends to arouse you (you know of it, so you must awaken some of the times when you toss and turn....yes?), that, too, is bad. You need to be able to sleep for close to 2 solid hours at a dash so that you can run through the stages of sleep, including REM.
At this point, I'll leave off any further comment. I know some reading here have had to tame thrashing in bed, and I'll leave it to them .
Good luck. This is going to take some doing, the odd slip backward, but you'll get there. Persist. Persevere. It's worth it.