So far, the date of your car company is determined entirely by production, with model line more a consequence of whatever features you manage to cram into the process over actual explicit design. I think there is a better way of doing this.
In essence, the car company needs to be on charge of production, instead of production being in charge of the company. The company would design and research it’s own things, and you would be in charge of implementing it. When you start you would be given a plot and an initial car trim(s) that you would be expected to build at a budget. As the time goes on more and complex trims get tossed at you by the company’s design team, and you’d be expected to be constantly retooling the factory to adapt. Maybe you’d be tasked with suddenly increasing demand on a hit, or sharply cutting back because design created a lemon. Research won’t go away as you’d still need to research production techniques optimizations. Instead of just growing and expanding when convenient you’d be challenged to create a modular yet efficient production line that can adapt to whatever whims the car company that employs you comes up with.
Of course the current CEO factory manager gameplay doesn’t have to go away, maybe just being a different game mode. However I believe the addition of a constraining “employer” would make the game a lot more interesting, while also allowing balancing complexities of market demand and what not to be abstracted to “Let sales handle it”