The only real solution to this is for the icons to have a larger native resolution and have the "100%" default essentially be a lie to the player; anything lower than Maximum would have to be downscaled, or the script for resizing ui elements would need different size-native assets for each notch and would need to call for them. This might slightly increase the load on ram, vram, or CPU depending on what FFXIV uses to render the GUI, but it would certainly require a partial coding re-write, a new class of icons, bars, and fonts for each scale % notch, new fonts/text for all ui elements at each scale % notch, and of course space on your pc to fit 5 more sizes of every UI element in the game.

In short, I doubt there will be any fix for this incoming.