Seems Dolphin lets you add thumbnailers for new formats, but unfortunately, not override its built-in ones.
But it caches thumbnails in ~/.cache/thumbnails, so if you generate thumbnails with a script ahead of the time, in the format Dolphin expects, it will happily use that.
Example command line:
magick /home/jin/tmp/test.png -filter point -resize 320 -raise 20 -set "Thumb::URI" "file:///home/jin/tmp/test.png" -set "Thumb::MTime" (stat -c %Y /home/jin/tmp/test.png) PNG32:x-large/(echo -n file:///home/jin/tmp/test.png | md5sum | cut -d ' ' -f 1).png