I’ve not user Klipper in a long while, but would it not suffice if Klipper knew 'Spool has 100m Filament left, print says it takes 110m, throw an error/require user confirmation to continue '?

Definitely! Mostly looking forward to see how/if other printers (especially Klipper) will support it

I really dig that setup!

May I ask about the seperate mini trackball? Is that connected to the keyboard or how does it work? :)