my apologies if this has been suggested before.
I think it would be a good idea if you could have sub assembly lines.
e.g. a separate line that can build engines from all components ready to be put into cars.
a bit like creating electronic circuits in factorio and putting them on a main bus ready for future use.
loving the game so far.
there is a thread related to this topic:
There you can find the positive reply from the developer