I only surf, but I have 3 friends who kneeboard OBSF regularly, and I think all have tried some form of concave multifin bottoms.
Both Tim and Rich say the boards are good for smaller surf, with the deep double concave and side fins, compared to their normal 22.5" wide belly bottom to slight V out the tailed shapes.
So you narrow down the board to allow easier banking over, since kneeboarding doesn’t allows as much leverage as standing up, and it will be fine.
Both guys are much heavier than you, closer to 200 and 220 lbs., so maybe 21" wide is wide enough with all that bottom shape.
personally, i have found all the leverage i need without standing up, if a board has some flex, but i do see your point if a board has a little stiffness to it.
What fin attachment are you planning to use and how design the flextail? Fellow KB’er here wishing to know. I getting ready to make a flex tail fish myself…
Most of the kneeboarders here don’t bother surfing unless it’s at least head high and OBSF moves in faster than most places in California.
The kneeriders, 5 that I know of, range from 6’5" and 210 down to maybe 6’ and 199, so all are big guys who have lots of leverage to sink the rails.
Their normal boards are 22.5" wide Lis fishs, but when they ride boards with concaves, they can’t seem to hold the edge in for long, powerful bottom turns. Function of too much planing surface with the concaves.
I can’t tell you how much to narrow down, since I’m a standup.
But for sure, the standups who really lay out their bottom turns ride narrower than 19" most of the time, flat to V the preferred bottom shape for head high and bigger.
Not saying Bonzer’s don’t work, but you need to ride them narrower if you have the juice like we do.
Notice the standup Bonzer’s in Oahu. They are narrow, semiguns, narrower than the standard Pipe boards.