Congratulations! Amazing! Excellent design! In fact making the flat bottom as wide as possible is a (very) good thing as the airflow is sealed by the sides walls of the track. This drastically increases ground effect as it is less likely to swallow air from the sides (F1 uses vortices generators, but your solution seems way more efficient in this context). Sealing the wheels too may improve this ground effect slightly (but since the air above is turbulent, it is hard to guess how detrimental these 'holes' are). I love this machine, great job, really a big BRAVO!
I wonder if making the whole car into a wing profile could work (though mounting everything would likely be a nightmare), in theory you should be able to use the sides of the track as a sort of endplate to make the aero more efficient, having the rear of the mini 4wd smoothly taper down to a point might also help the ground effect and the drag coefficient of it. On the matter of moving aero, I wonder if a hinged splitter (which would be in contact with the track most of the time, and would drop once the front lifts a bit in the LC, with a rear limiter to keep it from going too steep) would work well for keeping the front down on the LC, maybe a counterweighted version could also work for the rear, but it might get a little heavy
