The VRAM and load time requirements are pretty insane for reeeeally small improvement in clarity/sharpness for these Factorio HD Age packs.

Isn't there another way? Maybe using restoration models to add-in detail instead of simply upscale? (I assume you are doing only upscaling comparing your HD/SD images)

Applying a bit of sharpen like https://github.com/martymcmodding/iMMERSE/blob/main/Shaders/MartysMods_SHARPEN.fx or other variants (Quint DELC, basic Luma Sharpen) would make things pop

Thank you for suggesting this, though, and I'll be sure to check it out next year.
The textures are not just upscaled.
I upscaled the textures with Gigapixel 8 and manually adjusted the Ki model and sharpness. The problem is that the more freedom you give the AI, the more strange artefacts appear. On the other hand, if the sharpness is too high, the textures start to look extremely unrealistic.
You can see the result of this balancing act in the mod.

I would consider Topaz nowadays on the "low end" side of available upscalers.

I suggest to try some recent models, for example from https://openmodeldb.info/?t=pixel-art

I have tried over 30 current models from the database and the results were always worse or did not cope with the transparency of the textures.
These models were my starting point, then I tried out various test versions of paid tools and achieved the best results with Topaz Gigapixel 8.

Right, the transparency. I would say you could set background color to some color that the texture does not have and then clip it back, or just add a mask that you can cut back away (seen some nodes in ComfyUI). The first option would increase the work substantially while the second one should be automatic once set up.

I have a few questions for the sake of interest.
What kind of monitor are you playing on (1080p, 4k, ...)?
At what zoom level do you no longer recognise any difference between my mod and the original game?
What kind of jump in texture quality would you expect?

1. 34' 1440p QD-OLED ultrawide
2. it's not that I don't see a difference - zooming in I can see a lot of artifacts. Too crisp I guess (it hurts my eyes, really :D), the texture colors are too hard on edges of thin objects. Texture edges (alpha border) suffer the most. Zooming out makes me feel weird, like something is wrong and I can't really describe it
3. good question - probably would like to have clearer/less aliases (not sharpened per say, I despise ringing, clipping pixels or what SMAA does to edges, yuck) when zoomed in and then more pixelated (not smooth or crisp, but more raw, like a lowpixel game) when zoomed out, because things can be as small as my mouse cursor, which is custom - half the normal white windows cursor's size, so having hi-res textures is detrimental, as they start to kind of blend and smear all over themselves, paradoxically making image somewhat blurrier

It might boil down to my monitor size, it's resolution and OSD settings and how far I sit from it though.

As for the feeling that something was wrong, I also made a few mistakes by giving the AI too much freedom. It looked good at first, but when you looked at it more closely, it got all weird and confused. I can well imagine that people react differently to this, so the same texture might look good to one person and feel weird to another.
There is also a big difference between comparing a single texture in the editor and seeing it in the full context of the game.
I'll be looking at everything myself over the next few weeks as I play through the game, and I'm open to tryout other tools and things to get even better textures.