Final result is that apparenty, the guy who ran the shimmie (
http://shimmie.4chanhouse.org/, now dead) contacted the original artist in 2017 for a full size version of the art. The one posted to 4chan back in 2016 was
>>1478084, shrunken to fit the file size limits. The 5105x2583 version is probably the one that was shared to the shimmie. Because that site is dead and the Wayback Machine didn't capture the image, I can't confirm if there's a .png at that size, or if the artist only shared the .jpg I already posted. If anyone can confirm that, it would be amazing. That's all I can find at this point. Hope this was helpful for fellow data hoarders.