Until a vector pattern function comes along, I have adopted this method to create high res raster patterns. I don't think you can see the differences in raster and vector unless you zoom in really close, but who's gonna do that to your artwork. Anyway, my video shows a simple pattern. Enjoy!
https://www.dropbox.com/s/tnl1c7bun6lcuxw/Raster%20Pattern%20in%20Affinity%20Designer.mp4?dl=0