[glitch]

(I have not tried either of these solutions, so I'm not sure if they actually work.) Although still separate applications, Karabiner and Scroll Reverser are both free. (I mention this since you stated having to "pay for an app".)

(1) Karabiner: https://pqrs.org/osx/karabiner/ → Cf. Kellen Mace, "Separate Trackpad & Mouse Natural Scrolling in Mac OS X" (2014) https://kellenmace.com/automate-trackpad-mouse-natural-scrol...

(2) Scroll Reverser: http://pilotmoon.com/scrollreverser/

[kronie]

Wow, thank you for this. Scroll reverser did the job.