It gets better with experience. You can have a minimal base layer of common but rarely changing functionality. You can reduce static inline functions in headers. You reduce data structure definitions, but put only forward declarations in header files. (Don't use C++ methods, at least don't put them in an API, because they force you to expose your implementation details needlessly). You can separate data structures from functions in different header files. Grouping functions together with types is often a bad idea since most useful functionality combines data from two or more "unrelated" types -- so you'd rather make function headers "by topic" than putting them alongside types.

I just created a subsystem for a performance intensive application -- a caching layer for millions or even billions of objects. The implementation encompasses over a 1000 LOC, but the header only includes <stdint.h>. There are about 5 forward struct declarations and maybe a dozen function calls in that API.

To a degree it might be stockholm syndrome, but I feel like having had to work around a lot of C's shortcomings I actually learned quite a lot that helps me in architecting bigger systems now. Turns out a lot of the flexibility and ease that you get from more modern languages mostly allows you to code more sloppily, but being sloppy only works for smaller systems.