My thoughts on diplomatic priorities:
Kher-Ys and Muirgaythh don't actually need dedicated
diplomacy actions immediately - we can dedicate time to their projects in a decade just as easily as right now, and both of those projects will likely span many decades each.
Rather, the risk is that they, uh, just kinda die during the next decade while they're still small and isolated... but just sending parts of our fleet, if we can spare them, to support them is something we can do just fine without a diplomacy action.
Stel'iy-Rann, we're already effectively locked into given our turn actions for this half-decade. Beyond that, any hope of actual rescue is probably extremely time-sensitive... though I personally don't really hold out much hope for that.
Instead, this is probably just going to be putting us in contact with the DEldar faster than otherwise... which still has quite a bit of value, if we allow the information about the mechanism of the Curses and about Isha surviving to spread among them. This is still before the DEldar consolidated into a single (awful) faction after all, so we may well be able to peel some of them into at least being semi-reasonable trade allies of our bloc. (Presumably not the ones who got Nacretinei, tho
)
Lal'c-Tann is similarly time-sensitive, if we want to try to recover what we can from Terithillian before it is lost forever - at the outside, we might be lucky enough to salvage a second Forge of Vaul and a bunch of otherwise lost technological knowledge.
That said, salvaging a Major Craftworld's wreckage seems like the sort of thing that we'd want Vau-Vulkesh to be, uh, on-site for, so actually pulling stuff off there is likely to be delayed until we finish fixing our engines anyway.
I'll also say that I'm kinda leery of scrying for Terithillian until we get more Tzeentch curse mitigation done, too - Nacretinei is a much, much smaller deal than a Major Craftworld, so I'm less concerned about what we've done there.
Between those compounding factors, along with the fact that the Nacretinei search (and, potentially, sending some ships to look out after Kher-Ys and Muirgaythh) is already going to be eating hard into our resources, and I think I'm willing to let things lie for the next decade. We're certainly going to need to prioritize Terithillian within the next two or three decades, though.
Ulthwe is actually a very strong contender in my eyes - obviously, theirs is the least time-limited out of the offers, since Vau-Vulkesh is at far more risk than Ulthwe overall at the moment - but by the same token, even what to Ulthwe would be a minor token of appreciation would be sufficient to greatly shore up our position.
And, of course, having reasonable communications between craftworlds would be a massive force-multiplier in terms of the Diaspora's ability to actively collaborate on problems, conduct diplomacy, and generally solve issues.
Still, the lack of urgency does mean that we should probably delay it at least until the most immediately time-sensitive stuff gets handled.
Lal'c-Tann is a quick, simple, easy exchange that won't meaningfully take up our actions - and it's a relatively time-sensitive one too - if their fleets aren't repaired, they're gonna be in trouble. Fix their fleets, and they'll survive.
I'd also be inclined to spend some actions rebuilding their actual shipyards later on, once we can spare the effort - once they're in our alliance, those are shipyards that can churn out ships without using up our own action pool, to help protect Kher-Ys, or Muirgayth, or any other small and vulnerable alliance member.
Saim-Hann is a time-limited chance to curry favor with a major craftworld, while also gaining some combat experience with our new equipment and military setup in a non-critical situation where we'll have allies as a backstop.
As such, when it comes to diplomatic planning, I'm seeing something like:
Turn 5 - Stel'iy-Rann + Stel-Uit
Turn 6 - Create Diplomatic Council, use Scrying + Send Fleets to Kher-Ys and Muirgaythh to keep them safe, keep poking at Tzeentches Curse, build up military for Wyld Hunt
Turn 7 - Saim-Hann + Lal'c-Tann, with the remaining diplomatic slots either spent on continuing Stel'iy-Rann if needed, or on starting collaboration with Kher-Ys, Muirgaythh, and/or Ulthwe.
