Without knowing the other settings in place (ie behaviour of other fn keys) it's impossible to know if this was required or not.
Tech people are terrible for working out over engineered solutions to simple configuration issues.
(if anyone is wondering, the third-party app I speak of is Smooze, and it's a godsend https://smooze.co/)
finger-down-throat gesture accompanied by retching noises
At any rate, you can configure the "extra buttons" on most mice to do tasks on the Mac without special software. For example, I have a gaming mouse at home, and have one of the buttons configured to trigger Mission Control (showing miniature images of all windows open across all apps) merely through the Mouse pane of System Preferences. Also, games detect the extra buttons and can be configured to use them with no problem.
But yes, disabling mouse acceleration should be a built-in feature as well, and it is not.