Upgrading a codebase to PHP 8 is not an insurmountable task, I've upgraded our 1 million SLOC codebase at work in just a few weeks, with the help of tools like https://psalm.dev and our own strict coding standard.
Rector, which can automate these changes.
Rector is a very nice project, but I still haven't gotten around to integrating it into our codebase at work, because it uses phpstan instead of Psalm, and apart from being slower than Psalm, phpstan kept having various issues and crashes while scanning our codebase, unlike Psalm which mostly worked out of the box (I even became maintainer of Psalm, due to the large amount of additional performance improvements I added due to our needs @ work).
At work, we're currently using Psalm with a few extensions that make some autofixes as needed, a bit like Rector.