Author here.
I wrote a retrospective that might be interesting to read to some:
https://github.com/darthcloud/BlueRetro/discussions/289
In this article, I detail how my initial goal of leaning more about Bluetooth and sell a product turned into a software platform makers now use to sell their own hardware take base on my original design.
You can easily give the project a try without building a full adapter. If you got a dev board with the original ESP32 you can flash [1] the BlueRetro_pad_test.bin firmware, connect a Bluetooth device [2] to it and see the buttons inputs from a terminal [3]. You can also connect to the web Bluetooth interface [4] to take a look at the configuration.
To simply see the adapter in action you can look at my original Hackday Prize submission video [5].
[1] https://github.com/darthcloud/BlueRetro/wiki/Flashing-firmwa...
[2] https://github.com/darthcloud/BlueRetro/wiki/Controller-pair...
[3] https://github.com/darthcloud/BlueRetro/wiki/Getting-BlueRet...
[4] https://blueretro.io/
[5] https://www.youtube.com/watch?v=yj_Zbjb2_ms