Pomp cockpit set-up

So I’ve just built up a new pomp as a SS ‘do everything’ bike. I’ve put about 500 km’s on it so far and I’m not unhappy, but the soreness in my back if I do any more than about 25 km’s is problematic. This discomfort is occurring regardless of how much weight I’m carrying in the back pack, and even if I’m without a bag.
Having come from a mash to a pomp the geo and riding position is obviously different and I feel that the discomfort I’m experiencing on the new bike is because I’m bit too upright.

I’m running the metropolis handlebar (<none> | Full Speed Ahead) and they sweep back quite a long way, although are great on the wrists. I’ve already put a 130mm stem on. I guess chucking a straight bar on is an option but I’m hoping to keep the current bar set-up and put a set-back post on to get a bit more room, as it were.

Any feedback on the effectiveness of a set-back post in remedying the issue described would be most appreciated as at the moment my commute is a bit of a pain in the arse.