Cubeast looks pretty awesome, but it only works in Chrome, which is a non-starter for me. So a few months back I started my own Python/Qt app, which is roughly similar to Cubeast, but less features as of now: https://github.com/zwegner/CubingB

I haven't had much time to work on it lately, and there's limitations like only supporting Bluetooth on macOS and only one type of cube (the MoYu AI), but it's open source and works pretty well as a timer (both for smart cubes and regular cubes).