As the portal gets further in the distance, and the nether fog starts, the portal doesn't fade. (Pictured below)

I'm not sure if this would be possible as I have never actually researched anything about rendering, but wouldn't it be really simple to fix if instead of getting the portal to work with the fog, you just checked what the current fog distance is and what color the fog should be, and then overlayed a translucent texture over the portal to make it match the fog color & opacity (kind of like the nether portal texture when you turn that on). It's going to be far away since, you know, fog, so it wouldn't even need to be super accurate (unless you want to account for that one guy who will use max zoom with a zoom mod + a spyglass + minimum fov just to check how accurate the fog is once you update it).

I'm typing this while half-asleep, I hope it's not incomprehensible or anything lol

Hard to implement.