This was just a few KB over the limit but since PNG is lossless I recompressed it with pngout. The end result is a 15% reduction in file size so it isn't magic but it does work for the intended purpose. You can verify that it's identical to the original by decompressing both this and https://www.pixiv.net/member_illust.php?mode=medium&illust_id=56687764
comparing the hashes of both.
To decompress>pngout /k1 /ks /s4 /force input.png output.png
To recompress>pngout /k1 /ks /s0 /f5 input.png output.png
Might take a few hours, I have no idea as there isn't an easy way to measure execution time in Windows. I just left it running on my Windows server overnight.