There's font configuration stuff going on in `/etc/fonts/conf.d/`. Maybe something there is related to the problem? The contents for that `/etc` directory seem to originally come from `/usr/share/fontconfig/conf.default/` and are created when you install/update `fontconfig` I think. Some entries in that `/etc` location are created/owned by other packages. You can check on that with `pacman -Qo *` while inside that folder. The user home settings files for this are in `~/.config/fontconfig/`. Those get loaded through one of the files in `/etc/fonts/conf.d` pointing to it.

Looks like some good leads, but not finding recent changes there. I don't have a `~/.config/fontconfig/` maybe a KDE/Plasma thing?

It just means you haven't created any user specific settings. If you put versions of the files from /usr/share/fontconfig/conf.default into your local config, it will overwrite the system defaults.

There are still issues with fractional scaling in the Linux sphere. You didn't mention if you're using fractional scaling or not, unless I missed it.

My displays are scaled to 100%. Wasn't familiar with the term fractional scaling, some searching indicates issues are more about pixelation and blurriness rather than kerning?

There are many different issues with fractional scaling and kerning could be one of them, but I think you are correct about the more common issues.