Kroak slann and astrolith is only 4d3. From experience I can tell you that is not enough. You really want 5d3 because that way you are almost guaranteed at least enough points to summon a unit of skinks each turn. Yes it is a big investment. Summoning is supposed to be a big investment. GW does not want armies that take 2000 viable points and summon 1000 more on top of that. They want you to either replace the stuff you lose like slaneesh, or pay a bunch of points up front to enable summoning like us, so that it balances out. The reason to take the Oracle is flexibility. Slann are great but they are slow and encourage static gameplay. You only have 1 losat now, and you want to save that for more important teleports. A speed 10 wizard that can be used as an arcane vassal anywhere on the board is pretty useful. You are already taking slann so you don’t mind losing the spell, and the trog isn’t bad at bullying small 10 man units and taking objectives.