Edited by Videotape at 2018-08-23 13:52

The method mentioned before works well. Here's the link to the tutorial: http://forum.flymeos.com/thread-46186-1-1.html I've rooted my device without any problems and uninstalled all the apps that I don't use (Notes, Duo, Play Music...).
Edit: to uninstal system apps I used: System App Remover (ROOT) and I had to restart the phone to make the system realise the apps were gone .

Success on m6s

done?

Pretty sure you can just flash a supersu zip or magisk zip file through custom recovery, but you need to unlock the bootloader beforehand. (you can find the instructions on 4pda)