Thank you JimP for the link.

It helped me find a workable but so far not very elegant solution.

I inserted a textfield with the complete heading in front of every level 1 heading which I formatted as normal text:

{TC "1 Introduction"}Introduction

This way I only have the heading "Introduction" in the text and "1 Introduction" in the table of content.

This solved the problem to automatically get the right page number for the "(1) Introduction" heading in the table of content. That is an ok solution.

What still worries me a bit is:

1.) The tab-space between heading number and the heading in the table of content needs to be adjusted manually (I can no longer use the TOC-style to format this).

2.) The numbering of the Chapter Headings (Chapter 1,2,3...) as well as the numbering within the text field ({TC "1,2,3... Heading"} needs to be done manually.

The simplest way would be to let the number in the heading disappear instead of adding something. I don't know whether such a possibility exists.

Otherwise I am content with the above solution.
If anyone has some links to make the solution more elegant I will be happy to read them.

Thanks for the support.