OK, I tried that and xrandr now correctly shows several resolutions 
However, from what I understand, that’s the same result I could achieve by removing xf86-video-intel and then I get uglier widgets, as I said in Troubles with screenshots - #3 by lorebett
For example, this is a screenshot without using modesetting
and this is with modesetting (that is, not using xf86-video-intel):
I don’t think it’s my impression: the second screenshot is much uglier menus and widgets are not well spaced and everything looks much more compact (but ugly, IMHO).
Note that in Kubuntu, where, as I said above, modesetting is being used (without any configuration), the layout is still nice:
So I guess it’s really a problem somewhere in EndeavourOS…