Well for US it doesn't feel that the sherman comes that late because you only have to tech LT and you can go straight for major and since you get a free combat unit, teching doesn't even slow down your infantry aggression off course if you get light vehicles your sherman gets pushed back a bit.
So that's 50 LT plus 90 major plus the sherman cost 110 (I think) so 250 not that different from p4 or T34, with doctrines you can skip base upgrades by getting LMG on para or with infantry company, grenades are good but its not impossible to play without them and the most important grenade in my opinion AT nade is unlocked with vet so np, you do need the ambulance before the tank but that's a minor setback.
The puma does seem to come really early but so does the US Halftrack and I believe that's the main reason for the puma timing.
The Soviets T-70 coming late seems to be more a Tier system design flaw by having both light and medium armor in same Tier, this works for OKW having panzer II and Panther because of the huge fuel cost difference between them 55 to 175 and with 66% income,but T-70 and T34 is a mere 30 fuel difference with full income, in order to get an appropriate timing on T-70 the T-34 would come too early.
I actually suggest a crazy idea in another thread to switch T-34 with SU-76 making T3 all about light vehicles and making soviet T4 similar to US T4, I though about keeping the early game the same but making T4 unlocked only after you got 2 tech buildings up.
So you would open with T1 or T2 and unlock T3 tech again and unlock T4 this would mean obvious cost adjustments, you could go T1 T2 and straight T4 for medium armor or first get T1 T3 for light vehicles and then T4 for medium armor.
These are just my thoughts feel free to comment and point out anything I missed or not getting right
Sovjet starts with 50 fuel, US only with 15. I already subtracted this values from the complete fuel costs to make it compareable.
Interesting thoughts about your suggestion of switching tanks.