I heard of a few wikis and desktop apps which are FOSS, some has UI’s which look a bit old for, there are a few things like logseq I might try but from trying for a bit, i dont know how suitable it is for my usecase, but I want something that would be more specialized or at the very least have features that would be amazing for world building, (on a desktop app preferably but self hosted works too), like timelines, references to other pages, common stuff like Tags, Categories, and Taxonomies, graph view potentially, good search, templates. I don’t need all the features I listed, just some or what your think aligns with what I am looking for.
I like Zim. Used it for years. The big advantages you can have many 1000s of pages and it just uses a folder tree not a database, so you have direct and attachment access if you need it. Zim is a true hierarchical wiki not a simple notes app. There a plugins you can enable for more advanced features.
Zim does get slower with more pages for some operations like searches and some changes. I have one wiki with 4500 pages and do feel it is getting a bit slower sometimes. You can however just create another notebook at any time as long as your content has reasonable dividing lines.