Imo there is an issue with pace of the game also but it is not that Soviet light tanks come too late but that many unit come to early.
Progressively patches have made powerful units available earlier resulting in reducing the window of opportunity for certain units to pay of for the investment. To add to that bonuses have been added for teching last tier. The combination has created the "rush to last tier" meta.
Glad that we agree that Puma does not really have penetration advantage Su-76 even when fighting cheaper mediums.
Yes that the pace of the game change is the bigger reason that specificly soviet have a very bad mid game as only their lights got left behind i agree. Same with end tier bonusses.
It still leaves the question why fo soviet lights cone out so late when soviets have no ai upgrades nades or a good mg stock?
And though the su76 has better pen. Does it make the su76 a better choice then a puma? The puma offees far more valuable extras the a barrage for muni.