Arch is not basically gentoo. Gentoo is about as close as you can get to vanilla Linux while still being a distro. Arch is pretty opinionated about what defaults should run, gentoo is explicitly flexible. Also the compiling on gentoo thing really needs to stop, there have been binary kernel packages for ages, even before the recent improvement to binary packages. The gentoo installation in someways is easier than the arch installation, as long as you use defaults and customize after first booting up, and if you really want to customize stuff, portage is an absolute beast.

I saw a post a while ago that made the point that wine might end up being the stable Linux abi, we might come full circle

You need to set the scale in the sddm config file. I had the same issue where Kde properly set the theme but not the scaling. Here is a guide from the arch wiki

https://wiki.archlinux.org/title/SDDM#Enable_HiDPI

To find your screen dpi I think there is something in KDE display settings.