I just managed to update the software the maps remain at V5.
Mercedes is probably distributing the software V8 gradually by countries, it is probably not yet available in your country.
I think that's the reason why you still do not have the software V8 available. According to Mercedes the software update is free and supposedly forever. As long as the version fits on the 32GB cards !!!
I do not know if the availability is made by IP address or if something is analyzed in garmindevice.xml that gives you the indication to make the update available or not.