For a 20 handicap I would steer you toward T350 if you're looking at new irons. T200 will not be much different than what you have now. Most certainly NOT T100.

Handicap is a partial marker. If driver and putting drive your handicap then maybe the T100 would be appropriate. …But I doubt it. The T300 won’t be hurting your game over the T200/T100. Depending on your ball striking, the forgiveness of the T300 is likely helping you. T100 is definitely for better ball strikers.

I have always been an AP1 -T300 player. After getting fit, I ended up with T200 instead of T350. Primarily to gain a little more height as I’ve moved into my mid-70’s. I’m firmly in the camp that a Titleist sponsored fitting is hands down far better than an algorithm that doesn’t watch my ball flight or individual data from a monitor.

On line fitting tool can only get you a rough idea of where to start. Shafts will make a huge difference too. The changes between past generation T200, T300 to current T200, T350 is substantial so you must factor in those differences. You would be best off getting with a fitter and looking at not your age and handicap but looking at your clubhead speed, iron distances, and how well you make good contact with your irons. Good luck.

You really need to go to a fitter. There are no shortcuts to fitting and I certainly wouldn't do self-fitting.