I feel like the option for simplicity lies between "web component" and "make 4 pages". Something near "the button changes the CSS variable controlling the size"

I feel like the option for simplicity lies between "web component" and "make 4 pages". Something near "the button changes the CSS variable controlling the size". You lose out on pre-downscaled images but gain that the images look sharper for high DPI users and don't have to maintain the image sets or deliver multiple copies when the size changes.