What they did
The poster wanted to DJ outdoors through a Bose SoundLink Flex, but Bluetooth lag got in the way and the speaker's USB-C port only charged it. They used REA to analyse the firmware and Codex to do the rest: pin down the exact model and firmware version instead of trusting guides for other Flex units, find and switch on a USB audio mode that already existed, debug cases where the speaker enumerated over USB but stayed silent, and then build a macOS background service. The service picks the right mode and switches the Mac's output when the speaker is plugged in, and puts the previous output back afterwards. The poster did the physical side: listening, pressing the power button and replugging the cable when asked.
How it went
It works. With the Mac's Bluetooth turned off, audio plays over the cable, and the poster now uses it for outdoor DJ practice. They describe the delay as barely noticeable in their setup but say that is a personal impression, not a measurement. The code, tests, install steps and recovery controls to return the speaker to normal mode are public on GitHub.
Worth knowing
It is guarded to one tested combination of hardware and firmware (the original Flex on Main 1.0.20). The Flex 2nd Gen and other firmware versions are not supported, and the installer is Apple Silicon only and not notarized. It does not flash firmware. The poster's rule was that a command succeeding or a device showing up in Settings did not count as working; only hearing sound did.