What is the best Stack Exchange site to ask questions relating to all things juggling? Is there even a site that would be appropriate for a question like this?
It depends on what the questions are about. Some examples of the kind of question you would like to ask would really help.
If they involve or relate to the competitive aspects of the physical activity of juggling, I believe it would be justified as on-topic at Sports Stack Exchange (speaking as a moderator and long-term member there). We do not yet have a tag or questions about it, but we're happy to work on developing the relevant content.
If they are about personal or social aspects, and not the competitive activity of juggling, they will not be on-topic at Sports SE, sorry.
For your question, MSE is a good place.
My network-wide tagfinder query shows that a tag with the "juggl" substring exists only on two sites: the https://stackoverflow.com and the https://security.stackexchange.com . In both cases it has only a symbolic meaning.
Thus, we have no such SE sites.
But, I think, maybe the https://sports.stackexchange.com could be some similar.