I'm actually a fantasy cartographer myself. I don't have any resources specifically made for Affinity, but any brush sets that come with an image pack, can be used in Affinity too. Way back in the day I made a tutorial, but it comes with a download of fantasy map icons which I converted to a font. Anything in that download is royalty free stock made by me, no credits required. If you follow similar recommendations on DeviantArt you will find many other similar sets which include image packs to be used in any program. There's also KM Alexander who makes brush sets out of historical maps (Popple brush set) and he includes the pngs, so you would be able to convert them to use in Affinity. These are also royalty free, no attribution required. I'm not going to link to them all, just the one, but you can find them by looking through the recommended other sets or in his signature on the Cartographer's Guild, where we both participate as well. It's the most extensive online resource for digital cartography available and definitely the first place I'd look if you mean to seriously get into cartography.
As Affinity Designer is a relatively new program, I can't think of anyone who's made specific resources for them. I'm the only person I know of who's made Clip Studio Paint cartography resources, and as no one's ever expressed interest, I never packaged them up. It's not unlikely in the future I'll make some Affinity-specific cartography packs, and now that I know you are interested I will keep it in mind. I'm a professional cartographer and I am now using Affinity instead of Photoshop and Illustrator, in combination with Other World Mapper and Clip (I just love its brush stabilizer so much more than Affinity's, sorry!). OWM is a program you might look into to build the map base, because it comes installed with a lot of mapping features to automatically do what you want, but it's not free and doesn't do it all, though, so I'm using it in combination with Affinity for all of the photo-finishing techniques I use to push my maps to their final shiny state. No resource kit is enough on its own, in my opinion, without the skilled touch of a designer on top. There are more expensive programs specifically for cartography; I don't personally find them to be worth their subscription fee models so I won't mention those, but OWM is pretty low cost and I've been finding it works really well in correlation with Affinity, quickly becoming my new flow. If you only want freebies, I get it, lots of people do. I have a list of all the free fantasy map generators I could find. Not all of them would be useful for you, but some of them create interesting resources which can be used to reference your original drawing underneath. I made the list with the intent of giving to artistically challenged clients and people who want freebies instead of art commissions, and I think you could get a lot of inspiration from toying with some of them.
Anyway, those are all the cartography resources I can think of off-hand. There's a ton of great brushes with image packs if you follow these leads.
Now that I realize there's interest, I'll see if I can convert some of my resources to Affinity specifically. Let me know what sorts of things you'd be looking for. I have made far more brushes and textures than I have uploaded to DeviantArt, so I may have some I can pull out of my database and share if you can't find what you specifically need. KM's got some real nice hills and trees, so I think you'll be good in that department! But you may find some vacancies. I love making resource packs to share, so yeah, let me know and I can try converting some of my own symbols to be used in Affinity. You do mean as a brush set, right?
Since I'm sharing, here's an infinitely looping wave I just made the other day in Affinity. You are welcome to use it. I haven't released it anywhere else yet. I personally favor the expand, do a stroke, expand selection, do a stroke, expand selection, do a stroke method of coastlines, but this is a repeating pattern fill you could use to fill up water with wave I planned on releasing at some point. If you have any questions about fantasy map making feel free to hit me up.