Revert to Catalina

Hi guys,

My 2017 Mac desktop came with High Sierra installed and I have updated through Mojave, Catalina and now Big Sur. Unfortunately, Big Sur is obviously a step too far for my not-too-ancient machine. For example, Affinity Photo used to open quickly, now it takes up to 90 seconds or just bounces like a kangaroo on a bouncy castle until I force stop it.

I would like to revert to Catalina, (or even Mojave).

I have always used Time Machine, so I assume it should be easy to rebuild to the last Catalina backup.

How do I either get back to the last Catalina Backup or failing that, clean install Mojave or Catalina?

I will obviously back up all my data first.

I have been a very happy Apple user for the last three years but Big Sur has been a big disappointment.