$wpbd->insert_id is returning null

I have been trying to return a $wpdb->insert_id; but unfortunately, it is returning null;

That’s not true, you haven’t been returning $wpdb->insert_id at all.

        // insert 
        $toevoegen = $wpdb->insert( 
            ...
        );
        $lastinsertid = $toevoegen->insert_id;

You’ve been returning $toevoegen->insert_id, which is wrong. According to the docs $wpdb->insert will return either false or it will return the number of rows inserted:

This method returns false if the row could not be inserted. Otherwise, it returns the number of affected rows (which will always be 1).

false->insert_id is not valid and null. Neither is 1->insert_id.

Instead, return $wpdb->insert_id instead.

https://developer.wordpress.org/reference/classes/wpdb/#insert-row