I some weeks ago tried to convert a Drupal 7 site to Backdrop. As far as I know I followed the guidelines, but it was a disastrous waste of time.
I migrated the same site to D8 with hardly any problems, but I do not like D8. Several modules are not quite functioning (yet) and the whole construct is not to my liking.
I then tried to get into this forum for help, but it took more than two weeks to get approved - ridiculous. In that time I of course searched for other solutions, so, for me, Backdrop is not at all promising. Sad.
(I have several D7 sites in need of conversion before EOL (november 2021))