A lot of people say you need to go through at least three versions of something before it is mature - and v3 is not something you can design upfront. You need to see v1 both in code, and at runtime. Use it, get the feedback, and iterate. This is where AI tightens that loop immensely.

Lost you in the last paragraph - features are not "copy pasting because you already have all the building blocks" and "something may be twenty lines of code". Mid sized features often mean tearing up many layers of code across the stack to add in some sort of new capability. Tearing up existing code means there are all sorts of add-on considerations in addition to feature you are working on.