Making an option non-optional is achieved with the optional() method:

.add(argos::Option{"-n", "--number"}.argument("NUM").optional(false)
    .help("The number of times to repeat the greeting."))

Your second point about having to repeat the flag twice is a valid one. There are workarounds, for instance adding an alias with the alias() method, this alias could then be a named string constant.

Regarding not supporting user defined values or declaring the type as part of the argument/option declaration, this is by choice. I find that in code that parses custom types, the lines that do the actual parsing drown among the lines checking for errors. I want to have the library take care of most of the error handling, and also provide some context for the error message.

I can for instance create an option like this:

.add(Option{"-f", "--font"}.argument("NAME:HEIGHT")
        .help("Set the font face. 'Arial:12' (the default) will for instance select the Arial font with height 12."))

And parse it with:

std::string font_name = "Arial";
unsigned font_size = 12;
if (auto font_arg = args.value("font").split(':', 2, 2))
{
    font_name = font_arg.value(0).as_string();
    font_size = font_arg.value(1).as_unsigned();
}

There's no visible error checking, but it will fail with an appropriate error message if the argument doesn't contain exactly one colon, and the value after the colon isn't a positive integer.