Okay, I gave it a try and I made sure MixFix was working (console was running and the ini generated fine) but when I go into 3440x1440 for example, there's no change. But if I go below say, 2024x847, those resolutions will fill the screen fine and are probably serviceable enough for me.
Although with the file provided I didn't see a change unfortunately, and I did make sure to use the ASI file here and not the one from the original downloads, so I want to say I installed everything correctly (everything is in the games directory).
Although I'm not set with my own answer as I could have done something wrong although I did attempt this on a clean backup, although maybe I should try again?