Anyway you can take a picture of yourself while shouldering the rifle? When I’m using a PC, I have a tendency to slightly cant the rifle inboard. By slightly, I mean probably <15*. That said, when I shoot prone or other supported positions, I do not cant the rifle.
I assume the PC is fitted properly to you (front plate up at the clavicle notch and the rear plate up high on the back)?
Also, my work rifle has a basic M4 stock whereas my personal rifles have BCM/Magpul stocks. The M4 stock’s plastic “buttpad” slips easily but the others do not due to having a rubber buttpad.
Collapsing the stock can help, but assuming your running the shoulder straps naked, it shouldn’t be a huge difference as the JPC looks pretty thin in that area.
+1
good PC and plate picks
you have lots of real estate to work with going off pics; you may be one of the few that can run a buttstock the same with with and without a PC
most people will NOT be running the stock the same as without plates. it's more of a bottom/toe of buttstock to higher up on chest but below clavicle anchor point using the bottom toe of the stock as an up/down lever point for the rest of the rifle
canting is up to you. practice and shoot with and without, figure out what works for YOU. most of the guys that cant their guns have much less shoulder real estate than you do
I can't comment on length of buttstock for YOU, but you should not feel too stretched out, but definitely should be actively PULLING the rifle into your body, try the "C clamp" grip on the front end moving between targets, anchored in and pulling it shouldn't matter if you don't have perfect placement; not a fan of NTCH for fast shooting but that's a personal issue
the hypalon on the shoulder straps might be a bit slippery depending on your buttstock (I hate solid plastic), but they should not affect shouldering of the rifle like most thick nylon ones. that is their greatest feature IMO
as stated above, pull the rifle to your face, not drop your eyes to the rifle; faster and more consistent. not worth running a higher mount if you already have one, but something to think about in the future. I run lower 1/3 co-witness personally and it fits well for a "heads up" shooting position for my body type
if you plan to fight in a PC, you need to train in a PC; especially in fast reloads as it's a very different experience from slick; do it from behind cover kneeling, prone, supine, etc as you likely won't be standing up Nice and straight when bullets are flying your way
I'd consider getting different mag pouches or cutting the flaps/covers off the ones shown. Personal pref, but I'm less worried about losing my mags falling down a hill tumbling than I am about having faster access to the mags. If you have to reload your rifle.. you're almost certainly still in a fight
spend extra time practicing grabbing the one from the backside horizontal. unless you carry one on your support side hip I'll bet under stress that is the one you'll be going for first; fast reload mag on the support side hip is like your best friend if you use that. shorter 20 rd mags are easier to keep on the belt without banging into other stuff
bear in mind I'm neither LE or mil. just a guy that watches occasional Youtube videos so take all the above with a healthy dose of salt. advice from the internet is worth exactly what you paid for it