Keep up the good work man.
In this shot it looks like your backplate is just sitting too high on you.
True, try a MoW harness. Fabric ones tend to tug a bit more from what I hear but it's less work than having to buy more armour.Try a different harness that is NOT movie prop accurate. a cloth type! Get approved then wear the accurate one after that.
That type of harness isn't required to get approved. because its hidden and NOT shown in the movie.
don't stress on a detail like this. I spent years wearing a different harness then only after my itch to be more accurate did I make one.