Before reading this, keep in mind I’m not a lawyer.
I’ve seen games in the past do something similar, where you can upload a custom music file, and it will generate a rhythm game for you. I don’t see how this would be different. I don’t think there’s any copyright issue at all.
The only issue I can see is with Youtube (or any other video site you are using). Think about the situation from their perspective. You are using their bandwidth to “download” a video. What are you giving in exchange? If someone visits the website of that video, they interact with the content. They like the video, share it, subscribe to the creator etc, plus they get ads.
If you take all that away, understandably that platform will not like it. I wouldn’t be surprised if this is not allowed.
I don’t think Itch will do anything, as they are not harmed in any way.
If you really want to go for this idea, I would strongly suggest to get advice from a professional.