The likely issue is coverage, not just calibration. A proper equirectangular image represents 360° horizontally and 180° vertically. If your source is a single fisheye image with about 235° total coverage, or less than 180° vertical coverage, FFmpeg cannot magically create the missing view. Stretching that incomplete fisheye to fill an equirectangular frame will produce heavy warping, including the “squeezed” look you see near the bottom.

So before making custom remap files, confirm what the camera actually outputs:

• a fully stitched 360x180 panorama, or
• one/two fisheye views that must be stitched.

If it’s only one fisheye and it does not contain the full required scene, you cannot get a correct full equirectangular result from it alone. Custom remaps or Lensfun-style calibration may refine lens geometry, but they won’t recover missing field of view. In short: if the camera output lacks full 360x180 coverage, distortion in the conversion is expected.