Working with the pen tool, I've noticed that there is no way to snap the bezier handles to 45° and 90° angles once the node has been placed. Normally, this would be done by holding down the shift key while moving the bezier handle. In Affinity designer, this locks the handle to the angle it was last placed at.
My suggestion would be to keep the current behavior, but add an alternate keyboard shortcut that will lock the bezier handles to 45° and 90° (and possibly other) angles. Could be CMD + DRAG, CMD + SHIFT + DRAG, etc..
Thanks!