This exists!
The ASUS ROG Claymore II is an 80% "optical mechanical" with a detachable numpad. Mountain also makes several TKLs and an 60% (the Everest 60) with a detachable numpad. You could also make one using the KBDcraft Kit Adam and Kit Addams.
However, I agree with the others. I think you get the best build quality using a standalone mechanical numpad. If I was going to get a split design, I'd probably get an Ergodox or Keychron Q11 QMK or something similar.
I'm a frequent user of tqdm, but I've never seen that async example. Very cool!