Is there a way to rotate multiple photos at once in the camera roll? In iOS on the iPhone I only see a way to select one photo at once in order to then rotate it in the Photos app.

Is there a method or perhaps a third-party app that could perform this?
There's no way to do this without a third-party app or a shortcut at the moment.

If you have a Mac and you are using iCloud Photos sync, you can do it in the Photos app
If you're ready to pay for an app, OneEdit Pro will do the job
You can also use a shortcut like this one : Batch Rotate Images