You should be aware that a separate keypad has some limitations. Specifically, "alt codes" don't work when the alt key and numpad are on different keyboards. For example, on Windows you can use ALT+0233 to type é. But the alt key and the numpad have to be on the same keyboard.
You
could get around this by setting up an alt key on the numpad, but that's not great because it forces you to use two hands.
Instead, I programmed it in the QMK firmware so I can tap a special key (I call it "alt mode"), followed by the code, and then ENTER to exit "alt mode" and send the ALT+[xxxx] keystrokes to the computer all at once. It works great.
The IDObao group buy is live now:
https://www.idobao.net/products/ldobao-montex-pad-hot-swappable-mechanical-keyboard-kit?variant=39250453332099
That IDOBAO looks cool! I'm sure I could put that extra column to good use. But it might slightly limit the keycap options.