Cutting where nobody notices
The strip is resized to the platform width, then each row of pixels is scored by how much it differs from the row above. A gutter of flat colour scores near zero. Working back from the height limit, the first low-scoring row inside the search window becomes the cut; if none is found the least busy row wins and the slice is marked as a hard cut so you can add gutter space and re-run. Slices come back as a ZIP named in order. Everything runs in the browser; a very tall strip can exceed the browser's canvas size limit, in which case split it into a few parts and slice each one.
Platform numbers are the commonly published ones; confirm on the upload page. Background in Webtoon image size requirements.