Is there a way to save various Inkscape settings, like the letter-inches-grading and snap settings, so that it doesn't have to be redone every single time?

When Inkscape starts up, it checks to see if there's a file called default.svg and, if there is, it loads it and whatever document properties are set in that default file. This only applies to the Document Properties and not the options set in Preferences.

To set this default.svg in Windows 10, use this tutorial: https://logosbynick.com/inkscape-default-template

To set this default.svg file on Mac, use this tutorial: https://eastmanreference.com/inkscape-default-document-properties-and-canvas-size-tutorial