@Catorghans, starting a community wiki here with your response on the Backdrop Zulip chat (feel free to edit/expand):
Yep, this is a working procedure: first create a civicrm.settings.php in backdrop-root/
leave civicrm module enabled. Use the same CiviCRM version for Backdrop as you had for Drupal 7
run the Backdrop upgrade.
This also means that ...