The endplate does not change aspect ratio. Aspect ratio is wingspan squared divided by planform area, neither of which are affected by endplate geometry. The endplate gives you a higher
effective aspect ratio because in some ways, it has an aerodynamic effect similar to increasing the wingspan.
For F1 cars, at least the bit below the airfoil that extends far behind the trailing edge appears to be mainly for providing ad space.
But also, the taller you make and endplate, the less improvement you gain for each additional inch of height. F1 endplates are particularly tall because they're also the only structure supporting the wings. They have to extend down to the primary structure of the car, which tends to be placed quite low, whereas wing height is fixed.
And what FSAE cars have you been looking at? All the ones I see have massive endplates:
But for FSAE, what's more important, weight or downforce/drag? Big endplates add weight, and often on these cars, they add weight far above the CG.
"Because they do it in F1" is rarely a justification for doing any of the things they do in other areas of engineering because of how tightly constrained they are by the rules. Indy is different. FSAE is really different. Good things to bear in mind.
