one thing to be aware of is that adjusting the rudder rake on an H18 does not change the amount of helm on the boat by any appreciable amount, it mostly just changes how much your rudders pull against the tiller.
That's a great point, chang ing the hinge point of the rudder just changes tiller input force required, it doesn't really change the boat trim though (unless perhaps you raked so far forward that the rudders became unstable, which might not even be possible on this design)
Raking the mast, for example, significantly moves the center of effort of the sails, as does changing sail trim.
I forget about things I haven't adjusted in a while, but you are right, mast rake is probably the biggest change you can make that affects weather helm tendencies when you initially rig the boat. So @JoshCohen, if the rudders check out ok, you might want to rake forward one hole on the forestay to see if that helps! The performance manual has a median rake of 105 7/8" from the bridle to a point 48" up the mast but shouldn't be more than plus or minus 3/4" from that. It's a pretty narrow range of measurements that work, so measure accurately!