Problem solved: https://linearmouse.app/

Approximately the same features as SteerMouse but free. I've voluntarily donated to the dev, though.

This complaint was more widespread before the vast majority of Mac users were MacBook users. Today it’s a niche within a niche. I personally didn’t mind the expense even when it was a much less niche use case.

There are many ways to do it free on a Mac, too, including vendor-provided software.

SteerMouse fills a niche, it is not a requirement. Akin to, say, HydraMouse on Windows.

Yea but they're making money on simple things and you're not.