I know that for a flywheel design usually you use multiple wheels to keep momentum of the fly wheel so you can shoot at a consistent rate, what is the heaviest objects in vex to use instead of using a large amount of wheels. I was thinking maybe cutting steel plates in a circle and putting them outside of the flywheel to keep momentum or maybe cut lexan in a hollow cylinder and have pockets on the inside and put steel screws inside to keep momentum.

I built a flywheel in NBN and never needed to add weight to keep momentum.
We used a turret design so the flywheel was very compact, but we still used 2 omnis side by side.
If you dedicate multiple motors and take time tuning a PID controller (or something similar) you shouldn’t need to add weight for momentum.

If you’d like to add momentum without adding a lot of weight directly to your shooter, one thing you could try is to gear up your flywheel faster than your shooter wheel (ex. https://www.youtube.com/watch?v=aP9cql19RPM).