You didn't mention tight trees or chutes. Have you taken the board out in those places? That is generally where people run into issues with big boards
I have no tree experience. The parts below the timberline is wildlife reserve in our region and we strictly respect that. Oh, there's a short part between narrow trees to the hut but this hardly counts, its only some meters (20).
On a steep slope (30°) I can turn it within 2m in a longer line but I have no clue how long I could stay how narrow in deep snow with having real constraints. Backcountry, we've got mostly wide chutes with only short parts that are narrow. Theres only one long narrow brook valley of about 2m going seesaw I had to negotiate. That's all fine. But I'm the completely wrong person to ask, where the limits of this board are (or even other such boards) 'cause I only had few different boards and a more experienced rider can certainly handle far more difficult stuff with it. All I can say is that its the longest/stiffest board I've ever had and it feels far more agile/easy to turn than the shorter ones I had. So it really depends on the individual board, I guess.
and then... well... big... 158 is big for ME but actually its not really big