How to assign single-key system-wide shortcuts to numpad?
You can assign key commands specifically to a numpad/10-key, as macOS differentiates between the 10-key numpad &amp; the numbers above qwerty.  

Until [I think] Mojave, all key commands must have a modifier to call them, so you couldn't specify only a single number.
This has recently changed &amp; you can now assign a single numpad key to a key command. You still cannot do this with the qwerty numbers.

For instance, here I use  Ctrl ⌃   [10-key]  to switch between Spaces &amp; the OS is aware that using the same key combination on the 'qwerty' numbers doesn't call that switch.
You cannot tell by looking at the commands, they look the same, but they are considered different by the OS itself...
Just to demonstrate, I changed my 'Switch to Desktop 1' command to just be numpad 1 &amp; I can confirm this works as expected. Trying to assign only a single key from the qwerty section is not accepted.



However, unless you want to use some 3rd party structure like Karabiner [with which I have no experience] then you can only call items which either already have an entry in the shortcuts structure, or that have a matching menu command.
So, you can assign a single key to activate Dictation, as that has an entry in the Keyboard prefs, you could add * as your shortcut to take a screenshot for the same reason, but you couldn't directly assign
 Cmd ⌘   Tab ⇥   to another key.