if the dimples come out that easily, it essentially validates the whole process you're undertaking.
the brace position looks near where the heels will go or just pass over, so try to minimize brace depth to as minimum as possible and slope the sides so less heel grabber.

if it has to stay the same depth in ctr, see if the ends can be slowly reduced in depth down to 0 so plywood delam stress risers are reduced at the beam ends. i don't know if this will be an issue, but such a huge flat area is bound to flex under major wave pressure(let alone whitewater rock impact if ww or rockgarden) and if just one location doesn't flex, could be an issue over time. the shallower the better so flex is allowed. (even tho you have stringers at each seam location, you are not trying and shouldn't(inefficient) be trying to make frame construction.)

as in all whitewater boats(many others) esp. with flat/tish bottoms, the seat is a major structural and shaping element of the composite assembly. so plan that the seat bottom will be shaped flat over a large area to provide the same x-bracing farther back to keep the huge area flat even under load.(ie if still dimpled, put in temp beams while glassing, remove, make seat and see while sitting that the shape is ok. Probably will be.

however this is all crap, it works, looks fine and sure seems achievable!!
go for it.