Trying to download/export, and convert 1000ish kindle books to get out of amzns ecosystem

Does anyone have any experience with doing this? It’s not for myself, but for someone who isn’t tech inclined. I want to set him up with FOSS to avoid future shitification by azn. I know I need to strip drm, and then convert out of amz formats, but really have no idea how to do this. Any help would be appreciated! -j