I wouldn't use a widget for this. Just place the input field in a flex container with the buttons next to it. For better performance, you might want to try with nanoflows as this could well be client-side functionalitalty.
In the nanoflows, just pass the input parameter, do an empty check and increment the right attribute +1 or -1 (or whatever steps you need)
Best is to have two separate microflows OnClickPlus and OnClickMinus
You change the retrieve the object, change the attribute, add or subtract (with commit) and refresh.