Can I Fix Just One Folder Instead of My Whole Library?
Yes. Point it at any folder. There is no need to process everything at once.
Yes, and it is a sensible way to start. Take one year, or one album, and run that first. You see the result on photos you recognise before committing the whole library.
Running again later over another folder is fine. Nothing depends on doing it all in one go.
Takeout JSON Metadata Fixer takes a folder, a set of zips, or an Apple Photos library, and the free 500 photos is plenty for a first trial run. It runs on your own computer and is free for your first 500 photos.