If your seats and belts truly don't hold you in very well, you might consider better seats and harnesses. I'd avoid a cage, but perhaps a rollbar instead as this will make removing the top easier for "street" use.
Make sure its in tip-top shape.. replace any worn/soft rubber components (brake lines, fuel lines, oil lines, suspension bushings)
FWIW, I ran my SC with 23+ year old stock suspension for 1-2 years (on top of my then 10+ years of DE experience) just so I could learn what I wanted to address with the car. Plus, its fun when you are fast and everyone else knows the car is still stock
Beyond that:
1) Seat time
2) Autocross (yes, a time suck, but you can/do learn some mad skillz - one of which is learning how to learn a NEW course quickly which leads to #3
3) Drive tracks other than SUMMIT POINT. Yes its convenient.. but take the time/money/effort to hit up a new track.. VIR, Watkins Glen, Mid Ohio, Nelson ledges, Beave Run, Lime Rock, NJMSP etc. Variety truly is a spice!
I've run with other regions who were so tied to just one track that no one really knew how to "learn" another one track very well. Driving more than one also prevents the "ive driven this track 100 times" boredom that creeps in.
Until you know what you really want (maybe it won't be the car you currently have) I'd spend money working on the drover and then worry about throwing money at the car.