$140, lol. How about just selling the keycaps for an existing numeric pad or even small mech keyboard, and that's only if the physical keys are really needed.
The main slowdown I face as an occasional TeX user is remembering the TeX names of less frequently used symbols. For that purpose there is detexify (draw the character and find the name, doesn't work all that well) but a popup graphical menu or (if you must) a phone app would be fine.
Low tech solution here. Found a cheat sheet and printed it and stuck it on the wall next to my monitor.
Low² tech solution here: just google it.
(Or use Unicode search extension for Raycast—like I do—which is fundamentally the same as googling, but with fewer key presses.)