I am a very newbie of the project. I discovered it just today very accidentally. I was looking for such project literally for years. I am a software developer so I was even considering to write my own one, but I am glad I found your project instead. I was trying tons of galleries and none of them satisfied my needs.
I am very surprised this project is so difficult to find. I found it via googling the following request xmp HierarchicalSubject self hosted. But that’s just a miracle. I think you should invest some efforts into SEO to make your project possible to be found