I redid some tests on my breadboard, I definitely need 2 buffer at the output of the mixer (the sound deteriorates otherwise).
And I added a pot on my low pass filter 
I still have to do the tests for the EG/VCA, i definitely want to add a switch to be able to cancel the OH or not.
I already did the kick (the topic is here); I also did a snare but I haven’t ordered the PCB yet, I’m thinking of re-doing both later with what I learned doing the hi-hat.
Also here are my kicad project for the kick, and the snare.
I am still interested in anything that could help me to improve them