P: I don't want the thumbnail to darken when not selected in the Lightroom import screen

I agree. Unfortunately, people have complained about this for many years, e.g.

https://community.adobe.com/t5/lightroom-classic-discussions/import-from-folder-unchecked-thumbnails-have-a-vignette-effect/m-p/9733463

I think it's unlikely it will ever change.

In general, the Import window has very limited capabilities for careful examination of images and for culling. You might consider changing your workflow to import all photos, with the import option Build Previews: Embedded & Sidecar (faster) or Minimal (fastest), and the do most of your selection and culling with the imported photos.   Though you have a delay for importing, your overall time to finish the task and your frustration might decrease.

Make sure you've set the options Preferences > Performance > Generate Previews In Parallel and Replace Embedded Previews With Standard Previews During Idle Time.