ugoira-into-mkv

run it in a folder contains ugoira generated by PixivUtil2. will also look in subfolders.
fast, accurate, lossless.
obviously you need ffmpeg and mkvmerge.
you can change encoder in ffmpeg to any one that mkv accepts if you want it to be lossy but i definitely won't recommend any google codecs like webp, vpx and av1 because they generally sucks.