When you get close to neutral buoyancy, every detail counts.
I am 100kg, 55 years (in my experience, age is important for the ability to go narrow) but at a time my stable board for chop was a 110 litres 8'1" x 30", fish style (wide nose & tail). But I had a living hell on glassy water with a 110 liters board, but with a pointed nose and 27" wide...
Another example: I had 4 similar boards: 6'8"-6'10" all 120 liters of similar main width (29"-30")

the 3rd from the left is one of the stablest board I had. the 4th was quite hard. centimeters counts ...
I have now in my quiver a negative/neutral buoyancy board (105 liters does not float my 100kg + 7kg board + paddle + wetsuit), but which is manageable as it is wide everywhere (Tomo shape, 7'6"x 29")
me on it:
3rd from the left:



Note also that reducing the volume makes wonder for safety in hollow, powerful waves, but drastically reduce the early entry on takeoff.
So having a neutral buoyancy for a "kinda sim/fish hybrid" makes a lot of sense, but "playing in mush" may not, as the paddle speed and glide on weak waves will be noticeably impaired. For instance, my "Tomo" is no fun in weak waves, for these I would use 15-20 liters more (like the 2 boards on the left).
And for neutral floatation boards, be careful to add shape features that help a lot the stability (wide in the nose & tail)