If a feature request requires changes of such a magnitude it is important to break them down into smaller chunks that can be reviewed either independently or sequentially.

I have never rejected a PR before because no matter how bad a PR is, it always is technically necessary for the feature.

Define bad? If the PR contains lots of unnecessary changes such as formatting or renaming simply tell the person to roll them back and come again unless they have very good reason to do so.

If the code quality is bad, well, that's why you are doing the review. If all that matters was "Does it do what it is supposed to do most of the time?" some simple unit tests would be enough. Reviewing code means making sure it does what is supposed to do and does so in an acceptable manner. Criteria can be amongst others speed, security, ease of use or maintainability.

Insecure handling of inputs? Add sanitisation and resubmit.
Overusage of resource intensive features such as database queries? Group and optimize queries and resubmit.
The codes formatting is not according to the internal style guide? Configure your damn linter and resubmit.
...

If you don't want to close PRs outright you can request new commits that fix the issues you identified, reevaluate the PR and decide again.