The issue isn't you, the Osprey is just wank, literally nothing you can do to the vest, your gun or your stance to change that. I've probably fired a good few hundred rounds while wearing the Mk4 and it was always a contortion act of combative awkwardness. When it's stuffed with real armour it'll cover you well against artillery shrapnel and IED frag, which you care about in the relevant circumstances, problem is of course they aren't airsoft, and there's a reason cool SF blokes who jump out of helos in to room-clearing combat where every bit of speed and mobility count all wear American/American-influenced trimmed down plate carriers vs bulky mass issued armour.
My guess with the grey pad is that it's only intended to work in the prone, not sure why exactly, presumably just the result of troop trials, but it's simply far higher than you'd ever want to place the butt of your rifle when in any other shooting position.
There's a million and 1 different options for rigs, if we were all truly honest we'd say the most efficient of which is just a hi-cap in a pocket, but then all men would lose our form of fashion shopping and dressing up as a hobby. End of the day, if you're a small person, micro chest rigs and small belts are the best thing to go for; there are some specifically smaller plate carriers but they're not as common out in the market.