Might I suggest that the game use a different UP council algorithm than the current one? My suggestion is that the UP council shouldn't be so player-centric. So for example, the first race to meet at least 51% of the other races in the galaxy gets to hold the first UP council, and all other councils are determined from that from that point on. It gives an incentive for earlier exploration, and if the player hasn't got to meet a race in the UP council, then they aren't included in the UP councils, but all UP council decisions will be active for the player retroactively (so previous decisions of councils prior to the player joining will apply to the player once they join).
In fact, maybe add a late-game tech to allow the creation of secondary councils of races that do not wish to participate in the original UP council and so have those secondary UP councils start acting as an alliance or federation of sorts and create interesting diplomatic games and allow ensuing of galactic-wide wars.
I was going to suggest something very similar.
I agree the UP should start to meet after any race meets 50% of the galaxy, however, this plays into this question...
Race A knows Race B and Race C
Race B knows of Race A only
Race C knows of Race A, Race D and Race E
Race D knows Race C and Race E
Race E knows Race C, Race D, Race F and Race G
Race F and Race G know of just Race E
So in this case Race E, C, D, F, and G start the UP, however, wouldn't they wonder why other races they know aren't included?
Best Answer I can think of. UP requires one race after meeting more than 50% of the races to propose through diplomacy to join a UP, only one UP can be founded so which ever race proposes this first and has another player join is the UP. At that point any race that is part of the UP can invite additional members to join, or any race that is aware of the UP can apply for membership and have the races of the UP vote on including them. At this point any UP members would become aware of said race and have diplomatic options available.
What this solves...
Question: Why additional races that I wasn't previously aware of appear in the UP?
Answer: Other races that were aware of said race, invited them.
Question: Why aren't races known by Race B included in the UP?
Answer: They haven't been invited or applied for inclusion in the UP.
Question: Why don't I have diplomacy options for Race B even though they are part of the UP?
Answer: You would with this proposal as soon as they become part of the UP or apply for membership for you to vote on adding them.
Question: Why shouldn't their be two or more UP's
Answer: This is a peaceful general agreement non-alliance between many races, if races want to create alliances on their own they can but it's not the UP.
Question: Stardock has a goal of having games with 100 races, this would mean that 1 race could meet only a fraction of the races at 20 and still no UP?
Answer: In larger games with more races this number should be reduced to any race that meets 5 can propose a UP.
Question: Why isn't this game realistic
Answer: This is a game for fun and not based on reality.