Some tips :-
1. When holding down shift allow straight lines only and no curves. It's the norm in design softwares
2. When I drag to create the line, the drop point if off and not accurate to the nearest dot of my mouse position. I'm unsure if it's a bug or have you deliberately built a non-accurate system.
1. That's a good point, I didn't implement this shortcut. In my head, I assumed it wouldn't be useful because the editor only let you do vertical lines, horizontal lines and circle quarter. So making a straight line is pretty easy. If it was pure freeform like on Illustrator, it would be vital. 2. Maybe it's related to the editor only letting you do vertical/horizontal/circle quarter, right?
I tried to make a diagonal without a curve and faced this issue.
Also, I personally find the accuracy important. I understand that it would add complexity to the vertical / horizontal / circle quarter system you've built.
But as a guy who draws, it feels like unnatural when it over-extends.
Either way good luck !