For stunning colorful dot brushes, go to the tools panel of Affinity Photo and select the ellipse tool and create a circle by holding down the shift and drawing, add more circles and vary the size, add them close to the original design. Go to the swatches panel found in the view menu and studio and swatches and apply different colors to each of the spots.
Go to the layer menu and merge visible so they are all merged into a single pixel layer (as you cannot define them as vectors).
Go to the fx at the bottom of the layers panel and then set a 3D and outer shadow etc for the design. Go to the layer menu and rasterize and deselect the fx button.
Go to the brushes panel and right side menu and new brush from selection. Double click the brush entry and edit the spacing, size, goto the dynamics tab and set the size jitter to the max and set the rotation jitter to the max and likewise the hue and saturation. You now have your modified stroke. Apply it as you change the settings by going to the paint tool in the tools panel.