I haven't used Affinity "productively" yet for maps or anything else. But I'd say it is a software worth its price, especially since this is a one-time payment and no subscription. Of course if you are new to map making or drawing in general, there won't be as many tutorials for anything as there are for Photoshop, since that is around since so many more years.
I'd recommend to try some basic tutorials first if you are new to the program, so you get to know the tools and where to find them. And then you might try following general map tutorials that were made for other software. You may have to find out how to solve certain challenges in Affinity other than PS, but you should be able to accomplish most tasks you need for a map.

After all, it depends on your style anyway, if Affinity Photo or Affinity Designer is the right tool for you.