I'm not sure ALL the 'event cards' should be available to the players, since the goals of a diplomacy system is make the other factions interesting, not to make them act just like human players; they should have 'character' rather than just be out to win. However, I do agree that events/actions that are key to a faction flavor should be available to a player; the Phidi sending out some emigrants, for example, adds a lot to the in-universe lore of how they work, so it's something that players should have an option of doing. The inverse case of the AI Gremak empire buying slaves off their Marauder counterparts also makes sense, it's big part of what defines the Gremak in the game lore.
Overall I like the idea of race-specific 'event decks' and hopefully the event decks will get big enough you don't see all the events each game. While I don't think you need to go all the way to Civ VI's Agendas (which were imperfectly implemented), it would definitely make sense for the Yoral to have a totally different set of priorities than the Phidi and act like it.
As for what I'd want to see from the diplomacy system and the AI in general, a partial list:
1) The AI should have an idea if it's winning or losing the war, and accept a peace offer rather than always fighting to the death (this might already be done, I convinced the Ashdar to make peace in my current game).
1A) Some ability to extract/offer concessions for peace, rather than assume a status quo peace.
2) The ability to threaten an action; 'end your war with the Humans or I will attack' or 'stop buying slaves from the Marauders or I will end our trade treaty'.
3) Some sort of ability to lay claim to a system pre-colonization, or define some sort of sphere of influence.
4) Some sort of AI knowledge of your actions and opinions on them; slave purchasing is a good example of this, and should be expanded and other choices made that work in a similar fashion.
I agree in general every time there's a potential for friction with someone else there should be some choices to manage it; a choice that greatly eases the problem but costs a lot of Influence, a choice that requires a lot less influence but forces an action on the player, and a choice that doesn't cost much/any influence or change in status but will likely result in an AI action (war! Or something), and then possibly other choices as thematically appropriate.
I'm thinking for example there might be an event card for Tinkers that if another power has discovered a lost Tinker colony the Tinkers might demand their compatriots back so they can be re-linked to the all glorious Dizbix. The possible options might be-
1. Hold Summit with Tinkers to discuss the issue (costs a lot of influence, but satisfies the Tinkers until the Event Card comes up again)
2. Return Tinkers Home (Tinkers loaded on transports and sent to the nearest Tinker colony that can hold them, costs little influence)
3. Reject Tinker demands (costs no influence, major relations hit with the Tinkers)
4. Insist Tinkers must stay but allow a Machine Temple to be built on their world (thematic option, Tinkers are re-linked and a Machine Temple added, will cause other possible Event cards to be added to the Tinker faction event deck).