Primarily resort riding, we do get a lot of snow, but I have the odd groomer day. Mainly fresh snow, glades, steeps, natural features and terrain. Pretty much what you said the Berzerker should do...lol I just don't feel as fluid on it as I thought I would.
so what were you expecting....
idk abt the berz....but the charlie slasher is more of a deep powder cruising machine; it does ok on the groomers but with its rockered nose it doesn't have the bite for groomers.
as for flex...it depends on your weight but also your aggressiveness. the more aggressive the more stiffness you want...if you are too aggressive on a flexy board it will fold up or washout.
there is also torsional stiffness....now that I'm riding a billy goat...i think that the torsional stiffness plays a good part in how the board flows/fluid.
anyway its all about matching and compromises...it sounds like a billy goat is what you want but you got to be really aggressive...and it does ok for pow but deeeep pow a charlie slasher would do better...however perhaps a powder snake or a s rocker...due to the cs is flat between the foot and having camber there instead would help on the groomers.