Having the ability to customize the “choose and role” label

You can do this using hook form alter.

Ah, okay, but does it also work with the fieldset title?
I will try to read up on it, I am not very expert about this.
Thank you very much

I tried settings.php string overrides and it worked well (remember to clear caches):

$settings['locale_custom_strings_en'][''] = [
  'Choose a role' => 'Define your main activity'
];

From settings.php:

/**
 * String overrides:
 *
 * To override specific strings on your site with or without enabling the Locale
 * module, add an entry to this list. This functionality allows you to change
 * a small number of your site's default English language interface strings.
 *
 * Remove the leading hash signs to enable.
 *
 * The "en" part of the variable name, is dynamic and can be any langcode of
 * any added language. (eg locale_custom_strings_de for german).
 */
# $settings['locale_custom_strings_en'][''] = [
#   'Home' => 'Front page',
#   '@count min' => '@count minutes',
# ];

Oh...thank you very much,
used to using the ‘String overrides’ module when needed for various changes, I had not noticed that small changes could be set directly in the settings.php file.
My fault...
Thx, much appreciated

You're welcome, I was also positively surprised when I discovered how many strings can be changed with that method -- though there are some you can't.