This is a killer for me (Affinity Designer for iOS). I'm not an artist, I don't do freehand! I need the equivalent of CorelDRAW! functionality.
Suggestion: pencil tool fly-out (similar to smart shapes fly-out), with options freehand or straight (draw a straight vector from pen-down to pen-up). May be able to think of some more modes too... (but please see update below)
Work-around
Create a template straight line:
Draw freehand line (anywhere, preferably a clear work area);
Select node tool;
Drag selection box to encompass all nodes except two;
Delete (dustbin icon);
Drag selection box to encompass the ends;
Select the "sharp" option from the Bézier curve controls at the bottom of the screen.
You now have a prototype straight line. Copy anywhere you want a straight line in your drawing, then manipulate as required:
Select node tool;
Drag either end of the line to where you want it.
UPDATE....
I've now discovered this work-around isn't necessary. Simply use the pen tool rather than the pencil tool, and choose "line" mode - that does it!