It's not open source but I have to shout out Steermouse which has been replacing Logitech's awful Mac software for ~25 years
https://www.plentycom.jp/en/steermouse/download.php
This is my favorite. It Just Works with every mouse I throw at it and supports a ludicrous amount of button customization.
alt Hacker News
This is my favorite. It Just Works with every mouse I throw at it and supports a ludicrous amount of button customization.