Saddle / Seat slowly creeps backwards
Carbon assembly paste increases friction and can be used for metals as well.
Make sure to use a torque wrench and tighten to the torque printed on the seatpost clamp.
From the photos of the Fizik seatposts it almost looks like only the back screw is actually clamping the rails and only with 8Nm tightening torque? If nothing helps I’d get a seatpost with two side-by-side screws and ≥10Nm torque.

I put the saddle forward because the break in the rails happens at the
back, and I want to minimize flex.

Don’t compromise seating position just to avoid breakage.