In the past on large codebases I've used IDE plugins - Visual Assist X [1] lets you change function signatures, create declarations from definitions and vise versa, and add the include for a missing symbol. I'm no longer using Visual Studio proper though for C stuff so that's no longer available to me.
I recently stumbled on makeheaders [2] by sqlite's creator, used for the fossil version control codebase - it let's you write just the C files, and generates corresponding header files during build, with help from some macro keywords. Seems like a promising approach.
Curious what other solutions people have used to eliminate the unnecessary busywork of header files?
[1] https://www.wholetomato.com
[2] https://www.hwaci.com/sw/mkhdr/