I think shift is good while making a non-curved diagonal line.
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.
Loved the concept, makes drawing sooo easy even with a trackpad.
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.
What I can feel is that you aren't able/wanting to keep up with the rapidly evolving technologies. Maybe CTO and success has put you in a comfort zone. Maybe after being CTO of a crazy growth company you feel you've made it. Maybe what you're doing doesn't fire you up anymore.
Maybe you just got lucky and opportunistically became CTO.
You can either continue this stable, secure life and slowly decay or change course, whatever that may be.