I think the answer depends a bit upon your expectations for how much the different subject matters within your community appeal to a given member.
Do you expect a lot of overlap between the groups that discuss photography and cooking? If so, tags for these different subjects probably works well.
Or are you trying to serve multiple sub communities – one for cooking, and one for photography – within the same site? If so, you may prefer categories for each.
I’m going to make an assumption that you’re talking about a single community first, where people may together discuss many subjects.
My suggestion would be to start with something more like Approach B.
Having different categories for the different types of content will enable you to more clearly signal what kind of discussion it is, and what the expected behavior is of the participants. You can take that further in some cases by configuring the categories for those purposes (e.g. using the solved plugin for a Q&A category).
Then, lean first on tags for the subject matter. That gives you the freedom to apply multiple tags to discussions when there is overlap (photos of cooking!).
If, at some point, you observe it’s helpful to more clearly divide some subject matter from others, you can use the tags to help recategorize things.
I’m general, we recommend fewer categories, rather than more, especially when starting out, and a shallower hierarchy rather than a deep one. By default we only support two levels of depth. You have to go out of your way to turn on support for a third.
I don’t know that people heavily use URLs to prefill the composer. I can imagine it being useful in some scenarios but I would suggest leaving that idea aside until you find a need for it.
For discovery, something else to check out is the /filter page, which lets you construct more custom topic lists, which you could configure in your sites sidebar: Filtering topic lists in Discourse