i just finished a 6’3x21x3 that is to be used from minimal surf to chest high. it has about 2-3" nose rocker and 1" tail. i havent had it out yet. but if next weekend shapes up the way it is suppose to the board will see knee to head high.
you are correct that you get better paddling speed but when the waves get faster, taller, and more hollow the low rocker doesnt fit the curve of the wave. in contrast my big wave board ended up with a 5-6" nose rocker and a 3-4 tail rocker.
Maybe encourage surfinghigh for some hints/help? I have been riding afoafs simmons for about 3 weeks, and honestly, one of the funnest boards ever. Actually, probably the funnest. Goes well in everything, and has completely changed the way I look at backside surfing. I actually search out rights now, even when there are plenty of lefts.
I understand the hesitance to build one, but, and I will probably get flamed for saying this, take some of the elements of the design, and simplify what you need to. You will still have a surfable board, possibly and probably a very fun one for your intentions, too. Plus, if you start off with just some of the elements, you can work your way to a full blown replication over the course of a couple boards.
And not that you asked, but, i would say the wide parallel outline, the lack of tail rocker, and the flip in the nose, would be a good starting point and would make for a great board that would meet your desires.