If felt as hand-waving as reading that "Clean Code" guy.

Before you say "what about template files! jinja2! go templates! hcl!", that is merely a DSL, which is no longer a configuration file; it is effectively a program in a crude programming language, interpreted by an interpreter (the program loading the file).

It will devolve into messy "imperative" code writing "declarative" "configuration"? or whatever using templates, so there's no point in being a purist about The One True Way.