How to increment and decrement an output voltage by using two buttons?
If all you have to do is to read those 2 buttons, you could just forgo the interrupts and simply poll in the main loop.
The Bounce2 library does basically the same.

Also, about the PWM, why not an R-2R?

10V/0.5V + 1 => 21 steps &lt; 32 => 5 bits => 6 resistors.

But it could be more precise, as you would not depend on the oscillator, rather on the precision of the resistors.