How would I make a function to dynamically determine if user is logged to change navigation menu text?

I’m not really sure what you’re trying to achieve with the is_page parts – maybe I’m missing something – but what if you’d just write:

if (is_user_logged_in()) {
    $my_account = "My Account";
    echo ($my_account);
} else {
    $login = "Login/Sign Up";
    echo ($login);
}

return just returns the variables for use in your code, echo prints them to the HTML