It's not about splitting up the PR: it is about splitting up the _work_.
If you don't have feature flags, that is step one. Even if you don't have a framework, you can use a Strategy or a configuration parameter to enable/disable the new feature, and still have automated testing with and without your changes.
If you don't have feature flags, that is step one. Even if you don't have a framework, you can use a Strategy or a configuration parameter to enable/disable the new feature, and still have automated testing with and without your changes.