A practical approach is to avoid auto-trimming each frame independently. If the camera stayed fixed during the sequence, crop the first and last images manually to the same square size, positioning the Sun where you want it in the frame. Then interpolate the crop position for all frames in between using their timestamps (or frame order) and generate batch crop commands from that.

This gives you:

• one consistent crop size for every image
• predictable, even borders
• framing based on the Sun’s true motion across the sequence, not just the bright visible crescent

If you want software to help track the Sun automatically, video/compositing tools with motion tracking can do this well. A tracker can follow the Sun through the image sequence and keep it centered in a fixed-size frame, which is often easier and more accurate than relying on ImageMagick’s -trim.

So: for a locked-off camera, manual first/last framing plus interpolation is a strong batch solution; for a more automated workflow, use compositing software with tracking.