But if you want to shoot twenty-five missiles at a time, then the answer to "how do we balance this" is "make them wait about five more levels".
But you're only seeing balance as a matter of levels and CRs. There's a lot more to the system than that, and because there are more factors, the DM has the ability to allow things outside of the rules.
The DM can alter the action economy, increase the monetary cost, include the possibility of incurring a negative status effect, all sorts of things OTHER than just telling a player to suck it up and wait five levels.
As an advocate of rule zero, I often have to assert that Pathfinder (and games like it), outside of organized play, is meant to be a cooperative endeavor where the DM and players both contribute to a narrative. Balance is an illusion and blah blah gaming philosophy blah blah...