You really won't need even your 10.4 m2 NX4 to make a Serenity cruise along at good speed in light winds.
Will the Serenity give you the same "thrill" as you now get at a little higher windspeed on your FF158.....?
It's a little different kind of "thrill", but I found the Serenity to be more challenging than the Apollo last year.
The Serenity is never "hard to get going" because it slips through the water so cleanly.
So, there's no real need to pump or any of the things we do to get the wider boards with huge sails up and planing.
The Serenity does plane, but not in the same way as your FF158.
And, when the Serenity gets up over it's bow wave ( in as little as 9-10 knots of wind with a 7.5-8.5 m2 rig) it's very "challenging".
Tiny little shifts of weight or angle make a huge difference in the direction the Serenity goes, so it's a real challenge to keep everything stable when the board takes off.
A little weight on your heels and the Serenity will turn downwind 10 deg. or more very quickly.
A little too much toe pressure (railing) and the Serenity will basically "round up" or come up so high it stalls.
So, I think you really need to try one before you buy one.
Hope this helps,