"after a while I start to notice the same problem with it bulging into my lower back"
I believe I understand what you are experiencing here. I often experience this on my NeoAir. For me, I tend to sleep adequately on the NeoAir if I inflate it pretty hard, but it takes a while to fall asleep when it's inflated pretty firmly since it's not that comfortable. If I inflate it fairly soft so I sink nicely into the mattress, it feels comfortable initially and I fall asleep better, but then I sometimes wake up in the night with a sore lower back from it bulging into my lower back.
This problem seems to be inherent with all sealed chamber air mattresses, but it's easier to create with the NeoAir because it's thicker so there is a tendancy to inflate it a bit softer and sag in more. If you inflate the NeoAir firmly so you don't sink in very far, then there isn't really much point in having a 2.5" thick mattress.
Aside from inflating it more firmly, I'm not sure what to do. Lately I've been setting it medium soft, so I can fall asleep okay and then if I wake up in discomfort I add a breath or two and change positions…and hopefully stop the problem before it leads to more soreness.
For me, I usually sleep great on open cell foam (ie. those thick beige colored mattresses that are usually about 3" thick) but I don't believe this stuff is practical to carry for backpacking because it's quite bulky, easy to get wet and presumably heavy. If someone made a 2" thick open cell foam pad that was firm enough that I don't bottom it out hard, and the weight was reasonable then it might be a good solution.