@jlecoeur I have the exact same issue even though mine is with a tricopter frame. Each time I take off, the copter pitches forward. I just raised a topic here Tricopter pitches forward on take-off.
I have looked at the toml file which contains the geometry of the tricopter. It appears, the tail rotor is slightly further away from the CG compared to the other two. However, I am using the tricopter frame from RC explorer which uses a somewhat symmetric equal length of arms from the center. Asides extending the length of the tail arm, how else can I sort this out? can I reduce a scaling or something in the mixer? And if yes, how can I do this? Thanks in advance for your help.