Possibly requires a lot of thought and programming but could a semi intelligent storage solution be added between the body in white and paint processes (or after paint if people prefer)
This would allow better control of sequencing to different lines for model types.
I did think about this during the games design, because obviously in real factories, the drying process for car paint is much, much longer than we have it in the game, but in a sense, it doesnt change much in terms of gameplay. You will notice very quickly in the game, that regardless of how much space you have between two slots for cars to ‘build up’, the slightest imbalance will result in big queues anyway 
The best way to fix such things is to stagger quick fast-to-make models with slower, component-heavy ones.