[PufPufPuf]

I have one of those AliExpress ones that look like a white box with a colored circle on one side, stylized to look like a cat. They go for a few bucks used in the secondary market. I wrote a simple webapp for it to avoid using the bulky official app, it uses the web Bluetooth API (Chrome only sadly): https://purrint.github.io/

[Hackbraten]

What did you use to reverse engineer the BLE protocol?

[PufPufPuf]

Some people already did that, there were bits of Python and Processing code lying on GitHub I found by searching the Bluetooth device name. I just had to convert that to use web Bluetooth API and build the app around that.