The printPageMenu has also a option to show only subpages if available ("list-sub").

What three pages? If you use Zenpage as intended you have only one theme file for pages, pages.php, where you need to add the subpagemenu call. The function does not display anything if there is nothing to display.

I see the issue with the active class of the main menu. The function does indeed not "know" that you are using it as a separate top level function. Have to look if that is doable if set to toplevel display.

Meanwhile you will have to build your own function using getPages(), getParentPages() and the class methods.

thx for the answer
i mean right now i have to add the subpagemenu call in the codeblock, because i need a div wrapper around it. is there a way to determine if a page is a subpage and use it in an if-statement?

Yes, by using "getParentPages()"