What are some ideal conditions for 50% win rates in Among Us?

I don't think it's possible to quantify the conditions necessary for 50% win rates because the game has a non-quantifiable social aspect. In games like Chess every piece has a limited number of moves, which can be evaluated to a set number of points. In Among Us there is a limited number of kills, but unlimited variations of lies which can be told during an emergency meeting. This, coupled with the real-time gameplay means that the point system isn't feasible when every player's physical position and social cohesion is in flux.

I think that we can intuit variations of the settings to give an approximate 50% win rate: those are about the default settings. But changing this should be based on the skill level of the player group you're with, not on a point based system.

To give an example I set up an Among Us session with my coworkers, some of whom had never touched a mobile game in their life. These newcomers would step over bodies without seeing them and were incapable of telling a lie to save their life. No matter what kind of balance you create with the settings there isn't a way to quantify these skills in real-time gameplay. In the end, we tried aiding these coworkers by assigning two short tasks for a task-win but they were not able to argue fluently during emergency meetings - a skill that cannot be buffered with in-game settings.

There's a reason you don't typically point systems in real-time games like Call of Duty or Fortnite. You can evaluate players' skill levels against other players by comparing the two using leagues/ranks, but you cannot quantify the scoring system in games that rely on reaction speed and charisma in the same manner you would in a game with a limited number of turn-based moves.