57's are only 5 pop and thus spammable, I suggest an increase to 7.
The marder and the 50 mm's are PE's Anti tank guns, They are fine the way they are, Yes they are spammable yes they are fairly quick, But they have no turret to really take advantage of their speed, No armor or health to speak off and cant be recrewed, If anything marders and 50mm's should be cheaper than the allied equivalents, As you cannot recrew them.
comparing ATG and marder is quite useless, cus ATG have no mobility.
marder always can run away when they want