Shrunk down file size version of the original, from 1.4 GB to just 187 MB, while also processed to be lossless or at least maintaining its original quality.
About 57 images of the original is expressed in monochrome halftones, encoded in 24-bit RGB JPEG. By truncating the colors to just 1-bit (B&W), not only it saves space, it also removes the original's JPEG artifacts (since most of them exist within the boundaries of whichever color they resemble closest), turning them in to lossless images. Then saving them as WEBP images in only 2 color, they shrunk to just 5-10% of the original JPEG file size, while being at its best possible quality.
About 4-6 images are expressed in monochrome halftones, plus 1 additional color to express graffiti. Still using the same method, but with 3 colors (and PS to crop and paste the cleaned graffiti). This times as WEBP with 3 colors.
Nearly all the rest are expressed in monochrome halftones, plus varying portions of gradient grey. These are cleaned by making a base using the truncating method, then I superimpose the gradient portions on to the cleaned base, finally saving them as 8-bit WEBP. This cleans and saves tons of space compared to just converting them to 8-bit WBBP.
Q: WEBP?! Eew!! A: No, this gallery doesn't contain WEBP, only PNGs and JPEGs. Since WEBP doesn't have a good rep ATM, I converted all my WEBP versions to 4-bit and 8-bit PNG, which adds about 30 MB overall.
Q: Why is there JPEGs? A: Since all of the colored pages takes up more space in PNG than in JPEG, I just left them untouched, left to someone else to solve.
Q: WTF it looks like shit in full size. A: Adjust your image viewer to resample via any other method other than Nearest Neighbor (ie. High Quality Bicubic), or get ImageGlass.
Huge thanks to IrfanViewer, this is only possible because of its image conversion tools.