Shipped Mouse Keys 1.0.0, a small macOS menu bar app that turns extra mouse buttons into keystrokes.
By default buttons 4 and 5 type [ and ]; any of buttons 1 through 8 can be enabled and given a character. Public repo with a signed, notarized release at github.com/finereli/mouse-keys.
The reason it exists: Karabiner does this too, but it grabs the whole mouse through a virtual driver and re-emits every event, which makes the cursor lag on an older Mac. Mouse Keys instead subscribes only to button events, so cursor movement never passes through it. It types real key codes from the current layout rather than text, so games pick it up. Universal binary, runs on macOS 12 and up.