Someone asked my advice on how to best scan slides and film. This is what I said:
I've got a slide/film scanner that's been through use practically every month since I bought it in 2004. I don't know what the used market is like for those things, but two or three manufacturers were still making them the last time I looked around. They're (fairly) cheap and very effective – even exposure, even focus, and no distortion. Happily, there are two major software packages out there that you can use to work with any slide/film scanner on the market.
Less good are the flatbed scanners with a lamp in the lid and special brackets for holding your media. Much less good, actually. I have one, I don't use it, though I did when I shot medium-format (no choice).
Cheaper by far would be a reverse-mounting ring. But lenses behave very differently when reverse-mounted, and I expect you'd struggle with DOF and drastically uneven exposure and not immeasurable distortion. And then there's the issue of going through all of the mounting and praying that nothing moves, and sorting out the lighting, etc. You're effectively rebuilding a slide scanner! You will have decades of materials to scan, a dedicated scanner would be the most efficient and cost-effective in the long run.
I've never tried to use a macro lens for such a purpose, so I can't comment. But the last point above would seem to stand.
Lastly, you can send everything away for scanning. It's hazardous (slides lost en route, mishandled by poorly trained people, the company goes under) and expensive.