Font selection using key=val in custom article class

I am writing a simple class based on article that loads some commonly used packages and sets some defaults. The document font should be a key=value option documentclass{myarticle}. For example, font=charter, font=utopia and font=libertine set the font to Charter, Utopia and Linux Libertine, respectively. Charter should be the default (just passing font should not be possible). I am having trouble setting this up using the kvoptions package. If I understand correctly I should use DeclareStringOption[charter]{font}[] to set charter as the default of no font= is specified, but I am not sure where to declare the other options. In the documentation I see the function SetupKeyvalOptions{}, but I don’t understand what familiy, prefix and setkeys mean. The documentation is not clear on this (at least not to someone who’s never done this kind of thing before).

I figured I would put the lines required to set the font in some kind of if else statement. I have looked at the ifthen package but I don’t understand how to combine that with kvoptions. Also, it seems ifthen requires nesting of the different statements (correct?), which quickly becomes messy since I have more font options.

Here is what I have now for myarticle.cls:

NeedsTeXFormat{LaTeX2e}
ProvidesClass{myarticle}[2020/07/28 Custom article class]
LoadClass{article}

RequirePackage{kvoptions}

SetupKeyvalOptions{
    family = font,
    prefix = font@
}

DeclareStringOption[charter]{font}
ProcessKeyvalOptions*

iffont@charter
    RequirePackage[charter]{mathdesign}
elseiffont@utopia
    RequirePackage[adobe-utopia]{mathdesign}
else
    RequirePackage{libertine,libertinust1math}
fi

and main.tex:

documentclass[font=utopia]{myarticle}
usepackage[utf8]{inputenc}
begin{document}
    Content...
end{document}

The problem is that I don’t know how to evaluate the option in the if statement. Can you only evaluate true/false statements?

PS. I am also a little confused by the iffam command I see here (page 112). How does the combination between if and fam work?

EDIT:

I have changed myarticle.cls to:

NeedsTeXFormat{LaTeX2e}
ProvidesClass{myarticle}[2020/07/28 Custom article class]
LoadClass{article}

RequirePackage{kvoptions}
RequirePackage{etoolbox}

SetupKeyvalOptions{family = opt, prefix = opt@}

DeclareStringOption[charter]{font}

ifdefstring{opt@font}{charter}{RequirePackage[charter]{mathdesign}}{}
ifdefstring{opt@font}{utopia}{RequirePackage[utopia]{mathdesign}}{}
ifdefstring{opt@font}{libertine}{RequirePackage{libertine,libertinust1math}}{}
ifdefstring{opt@font}{lmodern}{RequirePackage{lmodern}}{}

ProcessKeyvalOptions{opt}

Changing the default in DeclareStringOption now changes the document font, but setting another option in documentclass has no effect.

EDIT2:
Moving ProcessKeyvalOptions{opt} to directly below DeclareStringOption[charter]{font} solves my problem. Still, I’d like to know if this is a "good" approach to doing this.