I think there's only really two real sets of artwork - the EGA and VGA releases. Some (EGA running on CGA or Hercules, VGA running on EGA) dither down the artwork rather than there being extra artwork for there, and the other versions are largely derived from whichever of EGA and VGA was a better fit for the target platform. Still a meaningful amount of work (especially reducing the 256 colour VGA art to 32 colours for the Amiga port), but it's the kind of thing that happened in games porting in that era.

Earlier this year I studied a few of the Amiga portraits and yes, they reduced VGA to fit in 5 bitplanes (32 colors), but they actually don't even use all of the colors. I found them to be around 20-25, keeping _more than a few_ aside for, say, the GUI.

Except in close-up chat mode, from memory all you get is text and a background color for the text, and not much cursor or anything else. So I'm still wondering why they didn't allow themselves a couple more color indices, which can go a long way even without dithering and anti-aliasing (which there isn't too much of in the VGA portraits). Maybe it ended up compressing a bit better.

It’s very obvious the Hercules was just a “we could” port - probably so they could use more machines for testing.

If you find a game that has a natural Hercules mode they often kicked serious ass, being at resolutions not achieved until VGA.

What sort of modern tools do you think would make it easier?

Photoshop? Or really any modern image editing software vs what was available in the late 80s early 90s