I don't want to end up locked into React, because frankly it's not all that. The likely replacement is Svelte but I'm not decided yet.
Yes ESLint can be is f***ing annoying sometimes, but when dealing with React hooks a second pair of automated eyes is very helpful.
IsFolder = true for libraries
CollectionType = "tvshows" for TV library... seemingly blank for Films/Movies
Use GET /UserViews not GET /LibraryFolders! The initial mistake was made based on the example given by @jellyfin/sdk. Jellyfin doesn't have any API usage docs, only specifications. To anyone else developing a client, always consult the Jellyfin web interface before anything else.