A fun analysis, Glenn. There are trade-offs in picking almost any gear option, and we most of us think and process this stuff at least a little differently. Sometimes in communicating about that, we can look at it from another angle and adjust what we're doing.

And sometimes that even works out well ... :-)

I think my path along this road was similar to yours. I've ended up in the camp of those who like the big, thick, but these days fairly light inflatables. So sacrificing brevity ... my thoughts on using a thicker inflatable:

(1) Give up 2" headroom in tent: probably a little less than that I think unless you keep it very fully inflated, i.e., when I sit up, my weight is in my butt, other parts of the pad become more rigidly inflated and my butt sinks a little. But nevertheless --- yes, it can be an issue. For me, this one is something that either is an issue with my particular height and the specific tent I'm using, and sometimes even exactly how I've pitched it ... or it's not. Generally for me it's not.

Let me add that once or twice when either through tent leakage or just coming in very wet from a deluge, when there's significant water pooled on your tent floor, it can be very nice to have a "taller" inflatable!

(2) Offsetting the possible height negative --- besides just the overall comfort factor, I like that I can pitch my tent on relatively rough ground that I would have avoided --- or been less happy with --- in a thinner self-inflator. What I sometimes used to do was also bring a ccf pad (at least for the torso) to layer under the self-inflator, but of course at a weight & bulk/volume cost.

(3) Not a good pad to sit on at camp or breaks. True, I carry a modest ccf square strapped to the top of my pack. I find this handier than a full rolled up ccf pad anyway; quick to deploy, I don't mind getting it (one side) dirty, helpful to kneel on sometimes for putting up/taking down tent, pretty light. If I want a backrest, there's often a tree, a stump, a trail sign, a rock, something to lean back against.

OTOH, I've hiked with friends who integrate a full-length ccf pad into their sleep system, and they love to toss that down and sprawl, lay down at breaks. So it's partly a style issue too; I don't take as many breaks, but when I hike with these guys I do. Sometimes I find I'm okay just laying on the ground when they're on their pads, but I admit that for such lounging a bigger ccf pad is nice.

(4) Have to have a full-length pad if it's one of the thick ones, as otherwise it's a big drop-off for your legs?
In fact, I disagree with this one. In winter or near-winter (shoulder seasons) I like having a full-length neo-air (and some sort of thinner ccf pad on top of it). But in warmer weather, my pack under my legs with the pack padding faced up and shoulder and waist straps carefully out of the way, the drop-off factor isn't much, and in warmer weather I sometimes have at least a little clothing or something to try to evenly plump up the pack. A little tweakier than using a full-length pad, but worth it --- not only lighter and lower pack volume, but less effort to inflate and deflate.
I do have a little thermarest neo air inflator that weighs 1.8 oz without batteries (takes 2 AAA batteries). I've only used this car camping, canoe camping, that sort of thing.

One factor in using your pack as sleep system padding is in rainy weather if the pack just gets soaked. I have a very light cuben pack cover to reduce --- or at least delay that outcome, but given wet enough conditions I am nevertheless faced with wet pack padding to lay my (down) sleeping bag on top of. This situation is why I believe that whatever deity you might believe in created thin black plastic yard waste bags. Okay, they might be useful for yard waste too, but they really fulfill their purpose and cosmic destiny on very rainy backpacking trips.

(5) Durability is also a potential issue, but with care --- not so much, I think. Two of my longest trips have been with original model neo-airs, and in both cases after a couple thousand miles of use they had seemingly un-fixable very slow leaks, but were still entirely usable. Just maybe have to wake up close to flat on the ground after a few hours and reinflate, or more commonly, when I wake up anyway (hey, I'm old ...), put a few puffs in to bring it back up. But that's adequate durability for me, barring anything unforeseen. And one is wise to bring at least a minimal patch kit for any sort of inflatable anyway.

So --- I'll stick with the neo-air based approach, but am the first to agree that there isn't some universally correct answer!