Thank you very much for listening and for the points of amelioration, really helps a lot !
You are right about the panning issues, at the end of the project I mostly listened to my tracks through my phone and speakers (instead of my headset) to check the mixing on different devices, but I could have checked again on headset to notice the panning inbalances.
And for the voice on the second duet track I only had either long natural vocal samples (that work okay for textures on other tracks) or this shorter one that feels a bit unnatural, maybe I should invest in a paid vocals VST to get something a bit more dynamic and natural (maybe you have some in mind that I would be very happy to check).
Anyway thanks for the constructive comment and for listening !