Thanks @Enigma644 for your answer.
Your software appears a lot during my research for a better solution than official muse app, I'll probably try it in the next few days, my 1st android app purchase
Is the visual feedback enough to work on mindfulness ?
Or is it possible/better to plug some sound effects (like in official muse app) using OSC ? (I'm not familiar with OSC, but it's probably easy to plug it somewhere on linux)