I am very happy for you that you want to step up and advance your sailing knowledge/experience!
As such, I'll give you some advice, from my own personal background/experience.
There are two kinds of sailors: Mechanical and technical. Obviously with your post you want to become a technical sailor. 90% of sailors are mechanical...they know how sails go up and down, they know basic sail trim, they know abit about heel, and mostly they are re-active instead of pro-active (take the tell-tales off of the jibs, and that will REALLY separate Mech/vs Tech!). But if you ask them "How do you think the main looks?" they have NO idea....
A 420 will help you with helming, but that's about it. Get an Etchell's (a Star is my first choice, but no active fleet around). You need a main whereby ALL the controls are at your fingertips..outhaul, vang, cunningham, sheet, even halyard tension...jib halyard tension, backstay, mast controls (Uppers and lowers), traveller. Race and race, learn from others. Pro-active reactions come very quickly.
I race and have raced with people that have been racing for a very long time, and are and forever will be locked into Sailing 103. And they are comfortable with that, which is okay by me. Happy to hear you want to be a "Really good" sailor versus a "good" sailor. Getting my first Star when I was 26 was the best thing I ever did to break me out of the mechanical mode. Not there's anything wrong with that...
Try to do fall frostbiting at Chicago YC Belmont this fall. The water is plenty warm. It use to cost 150 bucks for the 8 weeks of racing. Sail every Sunday. Free soup included after racing. Cost included use of a Club 420. You could also do a rent-as-you-go option. Jill and I did this for years. You need a dry/wet suit in the spring.
Give it a try. The RC is amazing, we would sometimes get 6 races in each day.