How or where can I change the projectile power in your code?

See the data pack system in https://github.com/Cannoneers-of-Create/CreateBigCannons/tree/1.18.2%2Fdev/common%2Fsrc%2Fmain%2Fresources%2Fdata%2Fcreatebigcannons%2Fmunition_properties.

Projectile balancing contributions are generally not accepted.