I've tried a couple times over the years to use Calibre to de-DRM my library. Previous attempts ended in failure. This community helped me find a way to fix my audio books, which inspired me to take one last shot at my kindle library.

I was able to follow this post comment and use a couple other comments to troubleshoot. Sorry for the Reddit link, but it's what worked for me. I'll include the text of the post in the comments below.

This method requires using Windows, but does not require a physical device as other methods that I'd found. The key points that worked for me we're using the correct program versions linked in this post and comments.

Another comment noted that changing the library download location in Kindle was a key to this outcome. I don't understand why that would be, but it turned out to be the case for me. The first try did not work, but once I changed the download location, deleted the old downloads, and re-downloaded the library everything went smoothly. Even the comic books worked!

I finally own the books that I've purchased over the years.