Camera calibration normally affects how future motion data is interpreted or captured; it cannot magically rewrite good gyro data into a file that never had it.

For the existing clip, first check whether your editor can apply horizon correction or reinterpret the original stabilization data. Keep the untouched source because a converted flat export may remove the information the vendor software needs.

Also distinguish a fixed horizon offset from a horizon that drifts over time. A fixed error may be easy to compensate. Drift suggests a different problem: bad motion data, a hard impact, file corruption or a mismatch in how the stabilization metadata is being read.

That diagnosis matters before you start keyframing a manual correction across the whole clip.

For the current edit I would test the original file in the vendor software and in VSDC before converting anything. If one application reads the stabilization correctly and another does not, you have a workflow interpretation issue rather than a bad recording.

For future shoots, recalibrate before an important session if you are seeing abnormal rotation or shake, especially after a hard impact or unusual mounting change.

The original behaves better in the Insta360 software than in my old intermediate export, so I think I lost something during conversion.

I will go back to the source and stop using the flattened file as the master. The new calibration can stay as prevention for the next ride.

That is the right distinction. Calibration is preventive/diagnostic for the camera, while the old clip has to be repaired with whatever valid motion/orientation information is already inside that recording.

Keeping the original 360 file gives you the best chance of doing that.