.NET c# service running as LocalSystem alters ProxyServer in registry, but browser's won't notice until (re)start

Question

I have a .NET c# service running as LocalSystem on Windows10 64bit. It's job is to alter the "ProxyEnable" and "ProxyServer" values located under

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings

in the registry. It does so, for all "human" Users of the System (that is, the key starting with "S-1-5-21.."). It works fine and all browsers the user starts up use this configuration (proxy or no proxy).

However, browsers that are still running won't notice changes (turning the proxy setting on or off) until a long time - OR until i manually just open and close the Windows "Internet Options > Proxy Settings" (without doing changes or saving them) or restart the browser(s). The latter two make the System re-read and propagate the settings to all (other) open browsers.

Now DllImport'ing wininet.dll and using the InternetSetOption does work

[DllImport("wininet.dll", SetLastError = true)]
private static extern bool InternetSetOption(
    IntPtr hInternet,
    int dwOption,
    IntPtr lpBuffer,
    int lpdwBufferLength);
private const int INTERNET_OPTION_PROXY_SETTINGS_CHANGED = 95;
private const int INTERNET_OPTION_SETTINGS_CHANGED = 39;
private const int INTERNET_OPTION_REFRESH = 37;
...
InternetSetOption(IntPtr.Zero, INTERNET_OPTION_PROXY_SETTINGS_CHANGED, IntPtr.Zero, 0);
InternetSetOption(IntPtr.Zero, INTERNET_OPTION_SETTINGS_CHANGED, IntPtr.Zero, 0);
InternetSetOption(IntPtr.Zero, INTERNET_OPTION_REFRESH, IntPtr.Zero, 0);

but only when i perform this piece of code as the logged in user. When the service performs it, it doesn't do anything. I guess it's because as LocalSystem the service is applying it to the wrong scope.

I have tinkered around a lot. Trying to start another process as the user is no option as this would require credentials. Also the service using LocalSystem is set.

I am just convinced that as LocalSystem, that is close to Administrator in terms of privileges, there must be a way to tell all applications and browsers, that the Proxy Settings have changed. Its just the propagation like opening and closing the Proxy Settings-GUI invokes, that i need to trigger somehow. But i don't know how. Do you?