Ok Tom, so for my soft harness I used the same principals as the accurate type. Two straps over the shoulders, like a rucksack and a waist belt, they all connect together with a back panel, metal or soft, the basic principal is the same. Using this arrangement I believe reduces the problem of the vest pulling up at the front (and down at the back - and helps with the collar armour choke problem.
It sounds like you are using a system like this. I would say is it tight enough? I like to get strapped in tight, with the metal harness it does mean that some movements are restricted, but not in any way to hinder trooping - just dressing lol, but we have all been there, thank god for good Mando Crew!
Have you tried with the pack more upright? I think your back armour may be able to sit more forward - based on the photos.
How heavy is the pack?
Lastly, you could try putting the harness on, then the pack for a few minutes, without the vest etc and encourage the harness to settle, then take the pack off and suit up.
Rob.