Ok, I think I’m getting around it all. It appears that FOTR and TTT both suffer from varying color and exposure shifts over their different reels.
ROTK is much more uniform, I’m thinking the print was just in better condition overall.

I’m in the process of correcting them the best I can in Resolve. Really broad brushes here, nothing too clinical, mostly just fiddling with the green/magenta balance and I’m only applying grades to whole reels, not on a scene to scene basis.
It’s funny going over those movies like that, one really realizes how funky they went with the grading! You correct one scene that seems too green and then the next one is way too magenta! It’s a fine line to navigate, I even purchased some original 35mm cells from ebay to project them as slides and have a reference, but those were shifted as well. And one can hardly use the home video masters as a reference, the grading is just so different in many places, plus the prints have a lot more contrast, which really emphasizes the grading shifts.

Anyway, here’s where I’am at the moment on FOTR. It’s hard to shoot for 100% accuracy without a proper reference point, so I’m mostly trying to make it feel right, tell me what you think: https://imgur.com/a/hOzQV5K