Hello DarkStar,
Seems like the problem is solved.
I followed the instructions from this video:
https://youtu.be/0F2yMO__KCk
I now have midi tracks that input to a single instance of the Aria Player which in turn outputs to different audio tracks with which I can control from Reaper's Mixer and add effects to.
Tomorrow I'll just make a 16 output clean project to save all those tracks as a template (I did a quick experiment on a 5 track template and it works like charm).
Thanks so much for your kind help !