Regarding clearing the output buffer, some time ago I had a similar issue and this solved the issue, at least for me:

    while (ob_get_level() > 0)
    {
        ob_end_clean();
    }

    ob_start();
    echo $toExport;

    $fileContent = ob_get_contents();
    ob_end_clean();

    echo $fileContent; 

Regarding clearing the output buffer, some time ago I had a similar issue and this solved the issue, at least for me:

    // Close output buffers till the level reaches 0
    while (ob_get_level() > 0)
    {
        ob_end_clean();
    } 

    // Start capturing output into a buffer
    ob_start();

    // Content to export
    echo $toExport; 

    // Get the buffer and clear it
    $fileContent = ob_get_contents();
    ob_end_clean();
    
    // Echo will start the download
    echo $fileContent;