If you purchased the commercial version of Affinity Designer, you also got a number of brushes along with it. They were separate downloads. Those same brushes can also be used in Affinity Photo!! I have already imported all of them into my AP and they work just fine!!
Here's how I did it. I created a temporary folder, went into Designer and exported all the new brushes to that folder. Only takes a few minutes. Then I imported them into AP from that folder. That was a lot easier than all the unzipping and such.
So, if you like brushes, you have a whole bunch of new ones that you can use!! I love how things are compatible from one program to the next in the Affinity series!!