For any curious onlooker: I tried JPEG-XL as well and kbin doesn’t seem to support it (not that that’s a big deal since the major browsers don’t support it either). To my surprise, JPEG-XL had worse quality than I expected for this image at the same ~300KB file size as this AVIF sample. It was still better than plain JPEG, but it was competing more against WebP than AVIF which is a bummer. Maybe JPEG-XL is better at photos than digital artwork.

Compatibility check: https://caniuse.com/?search=image%20format