I started the SUP to prone transition earlier this year. I still SUP on days where it’s glassy and there’s super long rides with a ton of paddling. But short of that, it’s prone all the way. If you’re a shortboarder at heart, you will be stoked at how much more freed up it feels without the paddle. It’s also easier and more robust in some situations: surface chop doesn’t matter, doesn’t matter how big or dumpy it is because you can take off in the white water. The pop up takes some getting used to, foot placement has to be perfect, but once up on foil it’ll feel the same just way better.
Board size: have tried a few different ones, and confirmed its volume that matters most of all. For example that freedom board above is sub 5’, but 3.5” thick (half inch more than typical), that buys you a bunch more paddling capability. I made my own board, boxy/chunky 5’x20”x3”, which floats me fine … and was surprised when I tried my buddy’s converted surfboard (maybe 6’4”), my board paddles way better! I have since made another 4’11”x20x2.75 (Armstrong copy) with more aggressive concave, deep chines, and narrow rails … I can make it work but the paddling is a pain. I figure I lost 5L of foam and it’s really noticeable. I have gone back to the higher volume cork.
Chine: mine have it. I see new boards are mixed whether they use it or not, and most aren’t as aggressive as earlier boards. I suspect it helps a little but doesn’t make much difference. Frankly I am a little cynical about the board designs for foiling, as once you catch the wave, board is just a platform above the water anyway. Seems kinda like fishing tackle, all the tricked out high tech features are more about catching fishermen than fish. ;-)