One of the oldest tricks is to do the initial upload, then change the submission file to get the full-size to work.
Frankly FA has been long overdue for a change on that breakable restriction.

The problem I face is when I change the submission file to do that very thing, it breaks the thumbnail. Images wider than 6000 breaks the thumbnail until you update the image with a size no wider than that.

Have you tried unchecking the "Rebuild the thumbnail from this file" box on the file resubmission page? I've never gone past 6000 pixels in width before so I've never tried it myself, but it might be worth a shot. An extra step yes, but it might yield results.

I have tried every variation on that. Updating the file without updating the thumbnail still breaks the thumbnail.

The thumbnailer breaks hard if your images use too much memory when decompressed. It's why they impose image dimension limits.

Besides, why are you pushing 600-dpi-plus images out on FA? I shrink mine down (I think the limit is 1280 pixels a side, but I can't see going past 2048 for screen viewing) and keep the full-rez versions for a collection CD/DVD ISO.

All the images are done in MsPaint (png file take less than 500kb, specifically this last one at 367kb) and it all worked fine until some changes a couple years back.

What I'm getting at is why the thumbnail working or not is influenced by the image size.

PNGs are compressed, and your art style lends to very high compression. 6000 x 4412 24-bit? Uncompressed is nearly 80 megs of RAM.

There are roughly 18 thumbnails for images on the front page, and those are pushed out rather quickly, so you can assume at least 18/second. It can take 3-5 seconds to resize a large (high-rez 600 dpi) picture down to a 200x200 thumbnail (120Kb uncompressed) -- the bigger, the more time it takes. FA's busy. So that system at worst case (everyone uploading print-quality pics) needs to handle 18*5*80 Megs, or 7.2 Gigs, of images at any second... and slap them on disk.

It's resource management, basically. You put limits in so that you don't chew up CPU and you stay fast.

In addition, I think the defaults for any web server setup prohibit taking up that amount of memory, no matter how powerful the system. I wouldn't be surprised if the IMUV coders put in detection of "ZIP Bombs" or otherwise require PNG/JPEG/GIF/etc files to stick to their specifications.

Which gets back to "why are you sending them out that size in the first place?" Creating them in high-rez is fine (and I'm surprised MS-Paint is that capable lately) . I save a master and a "FA" version myself. Uploading them for folks with sub-4K monitors to see makes no sense when even Firefox is shrinking them down to fit.

I'm used to gallery websites that show reduced, zoomed in, and full size. Deviant art had that and an option to allow people to download full size.

Got any thoughts on Weasyl?

Starting over in a gallery sucks especially when they don't have features that I've been begging galleries to do for years. Like a universal footer in submission author comment.