Resolved:
a) The place of the field for determining the user role is depending on the plugins/wp-roles-at-registration/wp-roles-at-registration.php.
The relevant line of the original version of that file is:
add_action('bp_after_signup_profile_fields', array($wp_rar_plugin, 'bp_choose_roles_registration_form'));
I substituted there bp_after_signup_profile_fields by rar_at_beginning .
b) Besides I inserted into MY_CHILD_THEME/buddypress/members/register.php between the lines:
<?php if ( 'request-details' == bp_get_current_signup_step() ) : ?>
<?php do_action( 'template_notices' ); ?>
the following line:
<?php do_action( 'rar_at_beginning' ); ?>
– and now the field at issue is displayed not the end, rather at the beginning of my registration page:
https://1a-spielwiese.de/registrieren/
PS.:
For information regarding the further reshaping of my registration-page cfr.:
https://buddypress.org/support/topic/reshaping-the-registration-page/#post-203615 [section 1.b) ff.].
