One of the microfilm/microfiche scanners at the university library where I work can also be used for scanning these slides. People often bring in boxes of them they've found in an attic, usually an elderly or deceased family member's vacation photos and the like. The scanner (model name is ScanPro 3000) can be a bit finicky at times, but is still easier and faster than what you've described in the note above. So if you're near a research library with significant microform collections, that could be a good option to consider in the future.