Lowkey wanna rename The T-60 to a T-45 variant since it's literally just a reskinned T-45 just Reskinned in every worth way. I like the T-45 way more than the T-60, but for an au of fallout4/the franchise is would still need to at least keep the T-60.
So I have an idea why not rename the T-60 to something like
T-45 Model Heavyweight
Designed for
-Better radiation resistance
-better Weather resistance
-more bulk
Compared to normal T-45
It would get its name from the War era as it was far simpler to simply modify the T-45 than to create a new T-Armor.
This would allow me to make my own T-60 while keeping the original T-60 armor without changing too much. Then that would explain why the east coast brotherhood has so many T-60 armor since in this au, it's simple just the heavyweight variant of their already existing larger quantity of t-45.
So to sum up
It's better than T-45 but weaker than the T-51