Hey, I know this is a year old topic, but I'm amazed that this hasnt been resolved already.
The brush tool needs a angle-constrained option from the last point of last stroke; modified by ctrl key.
Example: touch starting point; shift-ctrl to the next point; choose the snap-angle you want (I'd recommend going by 15degrees , but 45 is a must); touch end point and see a drawn line; release shift-ctrl. Voila.
I keep trying to come back to affinity from gimp for a year now and it amazes me that this functionality isn't programmed yet.
Cant work without this and changing tools while drawing just to get straight lines, makes me rather use other software
