I would probably think a lot of that would then have to do with how the display handles lower resolutions. Not every display, TV or Monitor handles varied resolutions lower than their native the same way - some better than others. Even if the lower resolutions are equally divisible by the total number of available pixels on the display.
But the point remains, you’re pushing your GPU to run a game at higher resolutions than its relatively capable of.
At the sacrifice of overall visual quality or being slightly blurrier, running at native 1080p with a lower preset will definitely get you the performance you’re looking for but not the fidelity.
You’ll need to make a compromise in some way whether you want to get an Xbox for MSFS, or a new PC if you want Native 4K at Ultra/Beyond Ultra.
With Xbox, you’re sacrificing both but may be better than what your laptop can provide.
With PC, you’re sacrificing money at the cost of a no-compromise native 4k at Ultra.