I've tried almost as many combinations as you, with similar results. This is what works for me as a side sleeper:
A foundation using a Thermarest Seat, 3 oz.
A REI Travel Pillow on top, 4.9 oz.
Neither slip, the REI feels good on the skin, and the combination is firm enough to support my head when on my side. And I can dial in the exact desired hight/firmness, as both are inflatable. Hollow inflatable pillows (Big Agnes, Kooka Bay, etc.) wobble too much, or aren't thick enough, or are too slippery. Down pillows compress way too much. Pillow systems can be quite heavy (12 oz. for the 2-piece Sea to Summit). No one has invented the perfect UL pillow yet--at least for me.
I tried the Z-Seat sitpad (instead of the Thermarest Seat), but it was just too thin and I lost too much head support.
There are two crazy things with pillow testing: when combining items to make a comfortable pillow, the final weight will be way more than you thought, and start to encroach on commercial pillow systems (Thermarest, Sea to Summit, etc.); and you can't test them in your bed as the mattress/pillow dynamic is different from your pad/pillow dynamic. Always test on the floor with your pad (and maybe bag).