I was wondering if it is at all possible to delete/remove every page that I have created from a wiki. I no longer wish to be associated with the wiki in question, and as such don't give them permission to use my original writing/content anymore. Even if it is possible (via FANDOM staff of some sort of takedown), do I have any rights to what I've written, or is it all made publicly available to the wiki under CC-BY-SA, and is their property now?
You can use Special:Nuke if you’re the admin manager of that Wiki; however, you should get advanced permission from all other admins to make sure they’re ready to start anew. If there’s personal information, then you should also contact Fandom Staff.
As a result of the license and other conditions in the ToU, once something is posted, users typically are not entitled to have it taken down. Even when residents of the EEA invoke GDRP's "right to be forgotten" (which is a law companies must abide by), the contributions themselves are left in tact. With the exception of content that violates the ToU (such as personal information on non-notable individuals), the best a normal user can do is send a request to local admins to have the content removed. The local admins may or may not agree to do it.
As an admin, you can use Nuke as Elder Pleasure suggested. However, the other admins (both present and future) still reserve the right to restore the pages you delete.