You can create a basic square brush in Affinity Photo via the brushes panel and right side menu (view menu and studio for the panel).
You can set it to any color, you can use it with the other options such as blending modes and opacity as well as symmetry etc.
Once you have created it, you can modify it. Select it in the panel and then go to the right and select the second button beneath the main menu of the panel and the editor will appear and you can then set the spacing, size, etc.
The general section is useful but the key section is the dynamics. You need to set the rotation to the maximum if you want to randomize the full range for the stroke. You can change the spacing of your shapes via spacing, have it as a complete flowing design or set to a higher value to have the shapes spaced out. You can also change the shape setting. Set to 100% for the full square but lower the value and you will end up with thin lines. Set the flow to add a more blurry or sharp effect for your stroke.
Go to the dynamics tab and set the scattering jitter to spread your designs throughout the image. Set the rotation to full rotation and random setting to make random rotations of those shapes. Change the size jitter to develop small strokes with low pressure and bigger shapes with more pressure. Set the hue / saturation etc jitter to vary the colors used, to get the maximum range set the values to 100%.
This works in an unusual way. Go to the base texture. It appears there is no nozzle. You can set a texture (select a PNG etc file). You can add and you can remove and also invert it. Set the mode to final or nozzle (though I can tell little difference between them). Set the scale up to 1000% but weirdly when you scale the texture upwards the texture actually scales down, and vica versa (which is somewhat confusing). The texture bitmaps can be color or gray scale but they always apply it in the current color and ignore the color in the original image. Apply the stroke and change the color and you will create all kinds of colorful textures applied across the image.
Go to the sub panel to add a round design (not sure why it does not let you add an additional similar design to the main one) and you can apply most of the settings to that round design as well. You can combine them in countless ways. You can also use it to add another bitmap such as a PNG file. You add multiple bitmaps to the sub section. Select the bitmap in the list and then you will see the options to draw at current nozzle or behind or end of the stroke. You can also use it to blend or erase. You can also sync the spacing and size but you can also not sync them which is useful as well. You can also edit all the bitmap settings such as add scattering, spacing, etc for those. You can also remove the added bitmaps as well as round strokes.
With the panel open, you can always try out your strokes and save that stroke at any point by using the duplicate and then working from that duplicate.