how can I link to a PlugIn admin-sub-menu page after processing a formular

Simply use add_query_arg(), remove_query_arg() and get_query_var().

On your page link, you use a simple query var like step=one.

$query_arg = add_query_arg( 'step', 'one', $_SERVER['REQUEST_URI'] );

Then, when the user processed the form, you just reload the page. Then simply check if

0 !== get_query_var( 'step' )
AND 'one' !== get_query_var( 'step' )

and process further. If you want to go back, just use remove_query_arg(), etc.