Speaking of diplomacy and spy networks, that system could definitely use a few quality of life improvements.
The most significant one being that having a diplomat building spy network in a country should also count as a resident, permanent diplomat for other diplomatic actions that are not dependant on time. So having a diplomat building spy networks also meant that you could use that same diplomat to, say, declare war, ask royal marriages, and all other one-time actions without having to stop building or sending a second one, but if you wanted to, say, integrate a vassal you're also building spy networks on then you'd have to send a second one because you couldn't have a single diplomat handling both "bars" at the same time.
The justification for this being that obviously your diplomat doesn't warn the target country that he's there to manage spies, for all they know he's there simply as a diplomatic envoy, and as such there's really nothing stopping him from taking five minutes from his spy business to write a letter to the royal palace saying "by the way, you're embargoed now".