IDEA-BRING BACK EMBASSIES MAKE THEM LIKE TRADE PORTS NEEDED TO BE DAISY CHAINED......
Create a model for an orbital embassy, make it so you have to build them from your homeworld and extend them (daisy chain - like trade ports) to the races you want to have relations with.
The diplomatic cruiser would travel between them like the trade ships do with trade ports. An enemy would have a target to destroy to weaken your diplomatic percentage with your allies or cause a current ally to declare war because your relations fell to a lower level.