The proper way to skip the dashboard for non-master users in a User Frosting site

I want any user who isn't the master user to be redirected to a separate page at /profiles.

I've achieved this by editing the admin sprinkle, adding this simple statement to the pageDashboard function in AdminController.php:

    if(!$currentUser->isMaster()){
        header("Location: /profiles");
        exit;
    }

I want to move this to my own sprinkle, but I'm not clear on how best to do this. Would I create my own controller that extends AdminController, and just replace the function? Or is there a neater way of doing it? What I have now works but obviously isn't future-proof as this file will be overwritten in future updates.

You can change where the users are redirected after login using the determineRedirectOnLogin service. See : https://learn.userfrosting.com/services/default-services#determineredirectonlogin. In your sprinkle ServicesProvider, simply overwrite the default service with something similar:

$container['determineRedirectOnLogin'] = function ($c) {
    return function ($response) use ($c)
    {
        if (!$c->currentUser->isMaster()) {
            return $response->withHeader('UF-Redirect', '/dashboard');
        } else {
            return $response->withHeader('UF-Redirect', '/profiles');            
        }
    };
};

You can then use the permission system to remove access to the dashboard for the non root users if you wish so.

Side note, like you pointed out, you shouldn't edit any core sprinkles and move that code to your own sprinkle.