Couple of things here. Without being able to see your PHP configs, I'm going to guess that they don't match from one version to the next. If you are using WHM/CPanel or some other control panel, you should be able to look through your PHP config on a version that works, print out all the PHP extensions it is loading and then make sure the 8.x versions have those extensions installed too.
You'll want to make sure you include the same "Disabled PHP functions" line from the older versions of PHP as well.
When you're done all of that and your site is running on PHP 8+, you'll need to go into your crontask and update the php version on the command line.