"Despite everything saying my PHP was 8.0"
Softac environment check or Moodle's environment check?
"the Softaculous clone tool set the PHP to 7.4 (inherited)."
Softac found the default lowest not supported version of PHP, then.
(didn't clone hidden 'dot' files? - hmmmm - tar command does!)
Conclusion ... Softac one button click didn't work! 
They advertise one button click ... and didn't deliver. Again ... hmmmmmm.
While I am glad to help and point you in the right direction for the issue, still kinda indicates a couple of things to me:
1. Softac Ticket System - where you should have reported the issue and have them resolve, must be closed on weekends. Or too slow to respond ... not that Moodle community forums (which are delayed) are any better on weekends.
2. Moodle's true environment check, needs to be run before files for higher moodle version are in place ... if that happens, moodle informs ... if done after higher version files are in place, it should stop the upgrade ... database won't be touched. OP then has a chance to replace the backup of the code directory taken just before the upgrade attempt and all will be well.
Not that any of the above will prevent you from using Softac. You probably will use it again ... but, please, do use Softac's Ticket System first next time ... have a feeling there will be a 'next time'! :| And ... schedule your upgrades for weekdays! 
Anyhoo - problem resolved and we can enjoy Sunday at least!
'Spirit of Sharing', Ken