If you use Export package, pls try setting custom headers and footers with call back:

$report->export()->pdf(array(
    ...
    //headerCallback and footerCallback could be js function that return header and footer content based on current page (pageNum), total number of page (numPages) and header/footer content set by pdf option or page-header, page-footer tags
    "headerCallback" => "function(headerContent, pageNum, numPages){
        if (pageNum == 1) return ''; //don't return any header for page number 1
        return 'This is a header' || headerContent;
    }",
    "footerCallback" => "function(footerContent, pageNum, numPages){
        if (pageNum == numPages) return ''; //don't return any footer for the last page
        return 'This is a footer' || footerContent;
    }",
))->toBrowser("myfile.pdf"); 

https://www.koolreport.com/docs/export/get_started/#export-to-pdf-header-and-footer-callbacks-(version-%3E=-5.0.0)

I'm afraid that with Export package's Phantomjs pdf rendering engine, dynamic header/footer content across pages is possible but header/footer height is always fixed. The best solution for your case is to export the first page without header/footer and later pages with header/footer independently and then merge them:

//export.php
$report->run();
$report->export("ReportFirstPageWithoutHeaderFooter")
...
->saveAs("cover.pdf");
$report->export("ReportContentWithHeaderFooter")
...
->saveAs("content.pdf");

//merge the pdf files here
...

There're multiple PHP packages to merge pdf files. For example:

https://packagist.org/packages/jurosh/pdf-merge

$pdf = new \Jurosh\PDFMerge\PDFMerger;

// add as many pdfs as you want
$pdf->addPDF('path/to/source/file.pdf', 'all', 'vertical')
  ->addPDF('path/to/source/file1.pdf', 'all')
  ->addPDF('path/to/source/file2.pdf', 'all', 'horizontal');

// call merge, output format `file`
$pdf->merge('file', 'path/to/export/dir/file.pdf');

Or https://packagist.org/packages/karriere/pdf-merge