Let's find why Naseem Shah might not be the best fit for T20I cricket. He's faced New Zealand the most, playing 7 matches against them. His bowling strike rate against them sits at 33.2, with an economy rate of 8.43. Looking at his overall T20I career, he's managed just 19 wickets in 25 matches so far, sporting a bowling average of 37.63, which currently ranks as the 4th worst. His career strike rate of 28.4 is also currently 5th worst.
One major issue seems to be his uncertainty about where to bowl yorkers and length deliveries and we can understand the reason for it from here that he hasn't managed to dismiss any batter via LBW in his T20I career so far. Since his T20I debut, both his bowling average and economy rate have steadily increased instead of decreasing.
Now what I think he should do is that Naseem Shah should focus on enhancing his bowling variety, consistency in line and length and should not worry about the pace as he should master Yorker and slower balls, targeting the stumps for LBW opportunities, building mental resilience because after being hit for sixes or fours he should not lose his momentum because I have noticed him during 4th T20I against England in recent 4 T20I series against England when Phil Salt and Jos Buttler attacked him he lost his momentum and control over line and length instead of trying different variations.