There’s a Panel Settings page, but it’s fairly barren. Is there a way to add options and such in there (and ideally save them as part of the site.txt file)?
I kind want the equivalent of the Kirby 2 “Site Settings” page, instead of having them be dumped on the homepage of the Panel.
No, that is not possible.
The Panel “Homepage” is in fact a merger of the former Dashboard and the Site settings.
But since you can add as many tabs as you want, you can dump that settings stuff into a “Site Information” tab or something like that. The stuff from this start page is saved in site.txt (if you have any fields on it).