If you sailed a Div. II way back when, I don't think it will take more than a few minutes for you to "adjust" to the Serenity.
It's quite stable and easy to sail across the wind, and on a close reach.
It's even pretty easy to sail when beating, if you "rail it" pretty much like a Div II board.
Due to the super long waterline length, the transitions may seem a bit difficult right at first, but your Div. II skills will kick in and you may find the "ZEN" of gliding along in almost no wind occurs with far less "learning curve" than you are anticipating.
It's just like all longer, narrower, boards with fairly round bottoms. You stay over the centerline.
I found the Serenity a bit easier to sail with a smaller fin or a weed fin.
The big wide Drake 70 cm that comes with the board probably takes you upwind better, but until you learn to control a fin that large, on a narrow rounded bottom platform, keeping the board form doing little
"snap rolls" from upwind to downwind can be challenging.
It's not like it drops you in the water, it's just that you are subtly "railing" the board and when you get close to flat the board kinda "clicks" over to the other rail so you have to be real careful when the board is nearly flat.
Hope this helps,