FitFov

Minecraft is designed for widescreens, so it normally only expands and shrinks your view horizontally. You see more to your sides on an ultrawide, but on a narrow(/tall) display, the image is cropped. You can try increasing your FOV to compensate, but your hand and your held items will still be cropped out!

Vanilla in 5:4

!Vanilla Minecraft screenshot in 5:4, showing a cropped view, with a held sword barely being visible on screen

This mod does a bit of math so that when your display is narrower than 16:9, the view will expand vertically by the right amount instead to compensate, which is much more comfortable.

FitFov in 5:4

!Minecraft with the FitFov mod in 5:4, showing an expanded view, and a held sword being perfectly visible on screen

Has no effect when running in 16:9 or wider, so you can drop this in a modpack to share across devices without having to change settings or anything.

Works on all 26.x versions, and includes compatibility with Iris. I don't imagine many other mods will have issues with this, unless they mess too deeply with hand rendering.