I do not agree with proposed solution but The request in the title “Support Numeric Keypad (+ / -) Zoom In / Out” is reasonable.
The same behavior works e.g. in Google Chrome.
I did not find zooming hotkeys to be customizable / listed in Obsidian/Settings/Hotkeys.
When user assigns Ctrl + NumPad1 to custom function then simply overwrite default behavior, i.e. stop zooming by it and start using it to triger custom function. When user restores original setting then start zooming by it again.