Wheel Choice for Flywheel Launchers

Agree, your rotational Inertia must be huge + friction, if you don’t kill a motor soon I’d be surprised.
We’ve gone for 1:21 gear ratio with single 4" wheels, we also set it up with as few gears as we can + a ratchet gear to limit motor damage.

agreed we made so many with the 5" and couldnt get it up to speed

There are advantages and disadvantages to wheel choice .Don’t just jump to change everything right away, test and see what works for you individually.
We use them and they work just as good as another one of our teams which uses 4in wheels