I actually like to use Comic Neue for personal stuff. https://comicneue.com/
Ubuntu is another nice font, which I like to use for more serious documents. https://design.ubuntu.com/font
For system fonts I use whatever comes preinstalled. I don't modify the font defaults in any way.
Outside of that the toolkit's file picker is used, as the system doesn't seem to provide one (via the portal), so the only reasonable fallback is to show the file picker that you know is there, which is the one of the application's toolkit.