It depends.
The F-5 is a great plane to fly and learn, and off course has a LOT less advanced systems and weapons.
Your learning curve on the F18 would be less steap I think, if you learn something more "basic" as the F-5.
I don't think it would be a waste, as it's another aircraft to fly and master, with more content. It's up to you to decide if that's worth it.