After some further testing I managed to buckle one of the spars slightly.
The expectation was that the board would experience the most strain at the points where your feet contact the board. And this is exactly where one of the spars buckled, as can be seen by the slight wave in the aluminium upright.
As this is essentially a beam under load, I suppose buckling would be one of the major failure modes. Prevent the beam from buckling and you get to rely more on the tensile strength of the next weakest component which might just be the rivets. Time will tell.
So I riveted some aluminium square tubing 12x12x1.2 (mm) to the existing spars. This resulted in a much stiffer and stronger structure, with an additional weight penalty of 400g, bringing the total weight to 3.35kg. This is still 400g less than my other board. Overall the stiffness of the board is now in line with my other boards as well.
Took it out for a spin at the cable park today - the first day it has not rained in the last two weeks. This time I had much more confidence in the board and did a couple of kickers, sliders and ollies.
The acrylic got some scratches from the sliders as expected but overall I'm pretty impressed with how well it has held up.
Next steps are to recess the rivets into the acrylic to provide a flat bottom. The cable park owner had some concerns about my rivets scratching his sliders. And then to mount some fins.
Some pictures below:
1. Buckled spar
2. Reinforced with square tubing
3. Size comparison with some other boards