Overall, it seems the greatest difficulty is that balancing sides and allowing people to fly with their squads are goals that pull in opposite directions.
That's why I liked the ENY system, only strengthened and allowed to go farther. The first ENY experiment didn't result in people switching sides to balance the sides. I think that is because the ENY limit didn't go far enough. People were mostly willing to stay with higher ENY planes than switch. They could still get some good planes, even by late-war standards.
The current system -- forcing a switch or a wait -- is sort of the extreme of what would happen if the ENY system were allowed to raise the ENY limit to 0 (where no planes are available).
I think an ENY-based system that allowed the ENY limit to keep being lowered, even all the way to 0, would provide a graded response that would give at its limit something equivalent to the current system, but it would allow those with very strong preference for staying in on the current side no matter what a way to achieve that.
Also, when one side has nothing but 1940's vintage aircraft available, a bit of side imbalance can be tolerated. It could be set up so that, at more extreme ENY limits, a side has only A6M2's, Hurri I's, P-40's, etc. -- no F4U-1's, etc.
If an ENY-based system that is allowed to go all the way to zero is not done, though, I think the current system would result in a bit more flexibility if it allowed for volunteers to be the first picked for switching.