Q: How do "clean" migration Aperture to Photos?

I had a few images in Photos 1.2, which I have since deleted.

It looks like the Photos library is still 36GB, though I'm not sure why.

Have you emptied the "Recently Deleted" album in Photos? Otherwise the deleted images will be saved for recovery.

To start, should I

1. Simply delete the existing Photos Library (username > Pictures > Photos Library)?

2. Import the Aperture Library (which I think would create a new Photos Library)

3. Delete old (current) Aperture Library (I believe this is OK? And that the "duplicated" new Photos Library isn't doubling up the space in my hard drive)

1. If you have all photos that might be in the Photos library elsewhere, yes, you can delete the Photos library.
2. You cannot import an Aperture library, because Photos cannot merge libraries, but you can open the Aperture library in Photos by dragging the Aperture library onto the Photos icon in the Dock. Photos will create a new Photos library from your Aperture library. It will try to save space by creating hard links to the photos in the Aperture library, so the new library will not need much extra storage.   See this link:  Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support
3. You can delete the Aperture library after migrating it to Photos, but I would wait, until you have tested the new Photos library thoroughly and you are sure the migration went well - no missing or corrupted photos, no Photo books you need to recover, no titles and captions you need to transfer.

See this link, how your Aperture library will be migrated:   How Photos handles content and metadata from iPhoto and Aperture - Apple Support