Lisa, If I may...
you have over 20 extra installed extensions, it can be one of them, in most cases old plugin is source of problems with new PHP!
SPPB 3.8 and Helix Ultimate that you have from us can work in PHP 8.0x without any problems.
Personally I have 20 private sites based on those two and they just work OK even on PHP 8.x.
In Joomla Admin > Global Configuration:
- Debug System: ON
- Error Reporting: Maximum
Then switch to PHP 8.0 and preview site, Joomla should display you more info about error source.