New Board Needed
Based on your weight!!!!! 
Thanks for that and yes you have pretty much hit the nail on the head:
“it sounds like you want more of an all-mountain board that can handle deep snow well, and a bit of park, rather than a strictly-freestyle-oriented board”
I will definitely have a look at those boards. In an endeavor on my part to understand the reasoning why recommend the 156 as opposed to the 159? Is it a standard weight/height issue or is it more my background with wake boarding?
It’s mostly a weight/board size issue (you’re only 72kg), and also I leaned towards the smaller size as you said you preferred the 157 to the 160 based on your previous experience (personal preference definitely counts for something). Also because of the shapes and profiles of the boards I’ve suggested, this alone will give you increased float in powder (compared to most standard freestyle boards), and this would be my main concern with a shorter board for your needs (float in Japanese powder).
The truth is I think either size would be fine. Shorter size is more manoeuvrable, longer size is more stable and gives increased float in powder. I think you can make the 156 or the 159 work. The 156 you will have to work harder in powder, the 159 you will have to work harder on groomers and in the park etc. Each has its advantages and disadvantages, but both are suitable, and YOUR choice based on having all the info and previous experience will probably serve as the best guide.
Overall, I would say the 156 would be more versatile, but the 159 will definitely handle deep Japanese powder with greater ease. You need to weigh up what YOU want to get out of your board and decide based on that.
Feel free to shoot through more questions.