One thing I noticed while riding my old Slack around - I believe the extreme rocker on both ends of the board makes it a bit squirly on groomers - my ass end had a tendency to slide out if I didn't concentrate on holding a line. I am guessing this is true of many pow boards.
While there's lots of variables I think you're right about having lots of reverse camber between your back foot and the tail making it washout. Like Supra, I'd recommend starting to look at something either flat or cambered in the back. Charlie, Barracuda, Family Tree boards, Hovercraft, etc. It's worth noting that even with the massive nose, the Hovercraft is only set 20mm back. Even the small tail should have plenty of contact with the snow.