IMHO you are worrying too much. As long as you stick to Emacs Lisp API and battle-tested libraries, the portability issue should be minimal. I've skimmed your config, and I think it can probably work out of the box on the other platforms.
You would still need to configure the external environment, (e.g. installing MSYS2 on Windows,) and address platform-specific issues, (e.g. you need exec-path-from-shell on macOS, and/or adjust default coding systems on Windows). Generally, these problems are easy to solve.
You would still need to configure the external environment, (e.g. installing MSYS2 on Windows,) and address platform-specific issues, (e.g. you need exec-path-from-shell on macOS, and/or adjust default coding systems on Windows). Generally, these problems are easy to solve.