I had written a Windows script for adding three-finger drag[0] that I use daily, perhaps it could provide some inspiration.

Basically, it is an independent subscriber to RawInput messages that only keeps track of whether or not to send three-finger drag, and posts emulated mouse messages using SendInput. I have a few other scripts that each run as independent userland processes that only monitors their own trigger and nothing else.

Tangentially, my TPMouse[1] script implemented inertia in a framerate-independent way so that it uses very little resource while having perfect simulation stability.

A previous discussion where I explained the analytic derivation for this low-resource exact-solution damped inertia can be seen in [10]

[0] https://github.com/EsportToys/PrecisionThreeFingerDrag/blob/...

[1] https://github.com/EsportToys/TPMouse

[10] https://old.reddit.com/r/Trackballs/comments/ym9q2t/tpmouse_...