OOC: how to have Jewish legions?
Roman ones weren't built in a day. They were a culmination of centuries of fighting
OOC: Well, centuries of fighting is certainly something Israel didnt lack.
Again, they needed a unified army, the confederacy might have worked as a way to avoid overcentralization with one tribe over the other but it required the grand priest as a religious-military unifying figure to work, something they didnt have as a monarchy with the monarch as a temporal ruler that the tribes could ignore even if he had divine right.
A Israel united militarily and religiously* would have the centuries of fighting needed to build their own "legions", assuming their numbers werent exaggerated, something that wouldnt be possible with the tribes squabbling between themselves, refusing to send soldiers or worse
actually allying with the enemy which happened many times, specially with the philistines.
Think of their situation as if the latin league was a loose confederation with Rome remaining a monarchy that collapses into civil war after the glorious rule of a David-like Rex while the people remain divided between followers of the traditional roman religion, hellenics, the cult of Baal, etc.
Rome would still have the
potential for greatness but would tragically never achieve it.
*Even if we say the Bible is VERY partial towards the cult of the abrahamic God, it is true Israel was a religious mess by then otherwise the book wouldnt be giving so much emphasis to "foreign cults" making God angry