Another great youtuber I follow named Michael New has made a web based tool to practice polyrhythms on:
https://michaelnew.github.io/web-piano
You can use a MIDI keyboard and it’ll show you how well you are hitting each note, here’s a video explaining how to use it:
https://www.youtube.com/watch?v=eCSxzV9ByZc
I’ve learnt a lot of the basic music theory I needed to do the Composition 101 course from Michael New, so I recommend his channel for that.
//Kjell