The Atomic Design book has a lot of good recommendations for setting up a design system: https://atomicdesign.bradfrost.com/

It does take a lot of work. But the idea is to build blocks (atoms) of elements, combine them into bigger blocks (molecules and organisms).

Design evolves too, and so these blocks should also be made to evolve accordingly. Bootstrap is a good example of doing this, but you don't really want every site to look like Bootstrap, which is where engineering comes in.