So, long story short;
1. Decided to upgrade Drupal version - I was on latest Drupal 8 trying to upgrade latest D9.
2a. Manually back up. 2b. Used Softaculous, backed up, then upgrade.
3. Upgrade done, site works at first, but errors when I try to log in or access update.php.
4. No problem! I got backups! Use softaculous backup to revert.
5. Site did not go back to its original form, shows installation page.
6. OK, a bit concerning, no problem! I got manual backup! Manually revert.
7. Site still shows installation page.