So nearly a year after I started this thread, I am really liking appendix carry for the P365XL. The big change for me was getting a Tier 1 holster - the Axis Slim. Expensive holster, but what a difference from trying to use one of my other IWB holsters for appendix. I think the hinge between the holster and mag carrier, and the bigger width with the mag carrier, just make all the difference.
It's super comfortable in all positions, including behind the steering wheel. Bending and sitting are much more comfortable than with the other holsters I've tried.
I'm carrying P365XL/appendix more than full size/hip these days when it's warmer out, and will likely do so through Summer. Right now I'm mostly wearing t-shirts with lightweight hoodies over them, and the rig just disappears. For the few weeks total of really hot days, I need to figure out if I want to carry against my bare skin under a t-shirt or wear one of the lycra athletic Ts under my t-shirt.
I'd like to experiment with one of my bigger guns, like a commander size 1911 or my CZ P-01, but these holsters are kinda pricey for doing carry tests.