Alright, ready to start this thing. It's honestly trivial in the grand scheme, and I'll be the first to admit that, but for a game that looks this incredible, I just couldn't stomach the atrocious LOD pop-in.
After confirming it wasn't some quirk with my setup, and was in fact universal - people apparently don't see it, lucky bastards - I found out FlawlessWidescreen has a memory patch that lets you increase the limit. Non-zero impact to performance, but at least geometry that's next to my feet isn't changing quality as I move. I still think it's a little surprising more people haven't flagged how aggressive the pop-in is. All 3D games have it to some extent, this case just seems unusually extreme.
Part of me really wishes this shit didn't bother me. But it's also kinda cool to be playing an even better-looking version of the game.
For posterity:
https://flawlesswidescreen.org/ - Main software is old, but the game-specific plugins have been updated. Simple installation, just needs to run concurrent with the game.