The TGI 90 is considered an S+ flex by KBS. They do the flex by the weight and they say the 90 gram TGI is the S+ flex. In 2019 they designated the 90 gram as an R flex, but now it is the S+ flex. The shaft never changed. Comparing it to the Steel Fiber S flex, the Steel Fiber is slightly firmer. One thing I would point out that if you hit your 7 iron 150 yards, you should be playing an R flex, not a Stiff flex if we simply base it on clubhead speed and that distance. If you prefer a firmer feel, or have a quick tempo, then an S flex might be OK. Still, with the physical challenges you outlined, more flex will always be more forgiving and would be the direction I would advise you go, or at least try.
I do like the Steel Fiber product and if the 90 gram weight is the weight you want, it does come in the R flex and would a good choice to try. Going from 120 gram steel to 90 gram graphite will be a transition. I don’t think it’s too much weight difference for what your needs are, but there will be an adjustment time from a feel standpoint. The only way to know for sure is to test. I would encourage building just a #7 iron to test before committing to the entire set. If it does not work out, then you’re only out the expense of the one club and can try another shaft.
There are some good lightweight steel options, especially in the Nippon line. Graphite absorbs shock better, but from a weight standpoint if you think you want to stay with steel, there are some sub 100 gram options in the Nippon line.
Britt Lindsey