I've been playing Halo: Campaign Evolved on the rather beefy Asus ROG Xbox Ally X without upscaling and things aren't looking pretty. Well, they are looking quite pretty, but they certainly aren't running pretty. You'll definitely want some upscaling to do that, and in my testing, FSR Medium is the best way to go.
Strangely, there isn't an option to disable upscaling altogether in Halo Campaign Evolved as of the time of writing. There are instead 'native' modes with TSR, FSR or DLSS. This essentially acts like anti-aliasing, without any of the performance benefits of upscaling.
