Are Shermans dedicated AI units? No they are not. They are versatile AI/AT units. Pzgrens shouldn't counter a unit like AAHT, but they should be able to counter Shermans. The main problem is high risk of One-shot squad wipe. One HE shot and BAM, they're dead. Fix this and they perform fine against vehicle. keep in mind that they are terrible vs infantry.
Please check the Pen of HE rounds. Then check the Pen of AT rounds. The best a Sherman gets is 120 Pen.In that mode they wont one shot whipe Pgrens. In HE mode it can as it has 0 Pen without this cooldown.
Jackson is an AT tank. Sherman is an AI tank that can help with AT sometimes through flanking or good RNG. Its primary claim to fame is AI. Its an AI tank. Because thats what its best at by design.
That is the best tank the USF has for cost imho because it is flexible. It can do both. One better then the other...