ya, getting jewels is the difficult part. I get mine by spamming nihlathak till i have roughly 20 crystals. then head into k3k and roll the jewels from the run into what i'm aiming for. Each jewel in k3k gets 5 rerolls before the next roll has to low of a level for the modifiers i'm looking for.
It kind of feels like astarte wouldn't come till the same time i'd want to make erawan. I aimed for erawan around 105-110ish when i'm just getting into sunless sea and such, when my defenses are still fairly low. I keep wedding dress up till that point, since the 6 io jewels + mo space for resistances, TCD, some other stuff. is really nice to have at that point. idk if Astarte is better there or not, since I feel the added defense from erawan is almost necessary, and i have 0 survival issues before sunless sea anyway (even in tu4's sunless sea wasn't much of an issue). So it kind of seems like finding jewels just to build Astarte at the 80's feels like it would be a waste of time since i speed through those quick anyway. at 110 is when i start kicking into gear with tanaan jungle dropping my sacred weapon/shield. Tanaan is easy as hell too. even Jitan isn't a threat to tu4 melee sorc, just takes awhile to do it properly (kite playstyle)