I haven't come across a method to toggle parts, chapters or sections directly, but you can get pretty close by including them from separate files, then calling

I haven't come across a method to toggle parts, chapters or sections directly, but you can get pretty close by including them from separate files, then calling `includeonly`. Perhaps also look at the `import` package: https://ctan.org/pkg/import https://ctan.org/pkg/import. If that is not for you, then yes, a custom environment (`begin`/ `end`) would be my next attempt. As for the engine, definitely use lua(la)tex, where -- perhaps -- the Lua integration can come in handy for this too.