As you see:
// this is how you set the font according to samples Font = Microsoft.Maui.Font.SystemFontOfSize(14)
And I have check the source code about the Microsoft.Maui.Font, the font family is read-only. So you can set it as a custom font.
In addition, I also checked the source code about the ios platform snackbar. There is no override method can custom it.
You can open a feature request on the CommunityToolkit/Maui repo.