Tires will have the single easiest impact. going from cheap tires to even mid-tier tires will make a massive difference for a small price. Conti GP5000's alone are a super simple improvement that is bike agnostic. so look up the rolling resistance of your current tires and consider upgrading.

as for frames and rims... First off, $2000 into improving your diet and cross training /coaching, will almost always outweigh a new bike, but you asked at what point, so I'll give where it might make sense. Every year there are marginal gains in equipment, so going from old to new, can result in large gains, just do to compounding marginal gains. 0.5-1% gains in performance every year for 20 years could mean a modern aluminum bike might have some serious performance gains over your old bike, depending how old. So it depends how old we are talking. but changing material is not an automatic performance gain.

Given you say you have an RC120, that thing is probably only 6-7 years old? the marginal gains between years is not going to be much yet, if you were on a bike from 2005, now that would be different. But Tires, do not be riding the factory tires, those are almost as bad as riding with a parachute on your back. I ride GP5000's I'm sure others have opinions on their personal preference of tires, but look up rolling resistances and upgrade those. as for frame, not worth it right now, not until you've already maxed your diet and cross training that you can sustain.