Over the weekend some further development came about on the soundboard front.
Suspecting that I might have left my soundboard to thick/stiff, a luthier friend of mine suggested trying out measuring the soundboard deflection as a more scientific approach to thinning it to it's optimum stiffness. He advised that on a classical guitar, a weight of 380 grams should be able to deflect a suspended soundboard 9-10mm, measured cross grain at the bridge location.
Doing the math, I should have aimed for a thickness of 2.2mm around the bridge area and about 1.8mm around the periphery of the lower bout with this smaller instrument. But, though these dimensions are thinner than what I ended up with, I feared it was just too thin, mainly because the top started to appear a little floppy, but also because I started to have difficulties hearing the pitch dropping any further at this point - It was as if the pitch had become a low rumbling indistinct sound. Mind you, I made these assumption with no practical experience to base my judgments on.
Loosely following his instructions, I quickly devised this crude and simple jig for this purposed. To each of the sides of the lower bout I secured a 5/8" think block of MDF with 1" plastic spring clamps, each block protruding 1/4" into the plantilla or outline. I centered the MDF blocks at the widest point at the lower bout, a little more than 1" below the bridge.
I propped up the soundboard on the jig, resting the MDF blocks in line with the bridge. A small MDF block supporting the top of the soundboard was also added to the upper part of the jig to keep things stable and level. I used big washers for weights and they proved to have the added benefit of allowing a ruler for measuring to rest inside them.
As this guitar is about 11% smaller than the standard, I scaled down the weight and deflection targets accordingly to 340 grams and 8-9mm.
The deflection test came out showing 6mm difference between the weighted and unweighted soundboard, indicating that it indeed is too stiff. However, I'm still not sure how accurate my measuring was. I don't know how much factors like friction between the MDF blocks and the jig played, how much the placement and size of these have entered the equation, measuring with the soundhole uncut versus being cut, etc. etc.
Nevertheless, I might have to revisit the thicknessing procedure again.