I use i3-wm and just set my laptop display and external monitor to their native modes manually with xrandr. Been doing it this way for years without an issue. The only time I’ve seen the output get chopped like you mention is with mirroring, where you have to use the lowest common mode - but I don’t mirror, I set each display independently as a separate output for i3 (but on the same X DISPLAY).
I also don’t use a login manager, I login to a VTY and startx, old school but simple and reliable.