I've skimmed this thread, so apologies if I have the wrong end of the stick, or am repeating advice above.
It sounds like your reloads and PPU factory shoot awful, but RWS factory shoots OK? I have never had PPU ammo shoot well in any of my rifles, and maybe your barrel doesn't like the 75gn Vmax.
Therefore try some reloads with the same bullet as the RWS factory ammo, or same weight bullet if you can't find the exact one. Try a middle of the book load at standard SAAMI length. Subsequent fine tuning charge and COAL may bring improvements. FWIW, I have never seen significant improvements seating bullets out long near the lands for short jumps. IMO this is benchrest twiddlyness that is suited to tight match chambers, though others' mileage may vary. I would assume that your Howa chamber will be large, and to get near the lands you have seated the bullet out almost as far as it will go. This could possibly reduce concentricity and consitent neck tension. I prefer 1x cal of shank seating depth.
Accuracy and consistency is fundamentally down to making sure your reloads are as close to identical as possible.
Well maintained kit and consistent shooting and is taken as read, on a windless day and without heating the barrel.