@vmassague Hi, I am pretty much doing the same thing and I faced a similar issue with you.
have you solved this issue?
https://forum.modalai.com/topic/919/using-motion-capture-systems-optitrack-for-position-estimation/2?_=1656684369036
Nevermind, apparently I looked in the wrong places:
https://dev.px4.io/master/en/ros/external_position_estimation.html has the answer: EKF2_HGT_MODE must be set to vision to use the voxl odometry instead of the barometer. Closing this.