After around turn 200 or so with Cravers, heroes started joining my empire every turn.

This is a balance issue on one hand: too many heroes means less sense of heroes to begin with and huge amount of micro MGMT (something that the game aims to elimiate in many places)

It is also at odds with the lore though:

1. They  are coming in also during crisis event when the lore states that the Academy is closed

2. There does not seem to be any limitation on their overall nor their max number per empire thugh lore states that there is a capacity as the hint for "Hero unlock progress" states: "Once the gauge fills up you get a new hero provided there is still capacity in the Academy"

So on my game in medium galaxy with 8 empires, the Academy already has 106 heroes at turn 223 thus the Academy does not seem to have a reasanable capacity set or it does not work

I would suggest that 6 heroes per empire are MORE then enough to manage, and shoot down endlessly in battles (as they are healed frequently and also respawn if a ships are left in the fleet)

Alternatively, the Academy can also have a capacity so that the empires are competing for places, but even then it should be: empire number * max number per empire and this limit should be less then number of empires * limit per empire

My Empire currently holds 32 heroes, and new ones are still assigned every turn