Have you considered JPEG XL for compressing images? It does significantly better (lossless) compression than AVIF and marginally better than webp for anime style and pixel art images. (AVIF does better at very lossy bitrates, but that isn't the use case here) See here for a comparison:
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It's still early days for it though and decode right now is slower (though encode is faster) than the older formats.
Also, are rpgmvo/rpgmvp essentially the same kind of wrapper for arbitrary file types, or are there differences between the way MV handles images and audio?
