next raid boss change

I think omastar will stay for a while, the first raid bosses stayed for months. As to eggys outclassed by raikou, this is only true against water type, eggys are effective against rock and ground as well. Also, how many raikous can you power up? You are limited by candy which is hard to get, unlike dust that you can always get more.

I wouldn't power up a bunch of grass just to solo Omastar, the same way I'm not powering up six Tyranitar to solo Alakazam: for me, it's just not an accomplishment worth the investment required (though I'm in no way implying that Ttar and Eggs are of equal value; I just don't want or need that many Tyranitar). I do recommend you power up at least one good double-Grass Eggs to add to your arsenal, however.

The ultimate question is this: Do you really want to spend half a million Stardust for a chance to catch a raid boss of limited value in the current meta?

IMO your have a ways to go before you can solo Omastar. The people I've seen that have done it have Eggys with CPs between 2200 and 2500 or more. Solar Beam is better than Seed Bomb, and that difference is bigger than Extrasensory vs. Bullet Seed (Bullet Seed is better against Omastar).

Eggy's aren't outclassed by Raikou, they are different. Raikou is not so hot against ground, that's its only weakness and many ground mon are doubly resistant to electric, but Eggy is great against ground. So not only Omastar, but Golem, legacy Rhydon and probably future Ryperion, and also there's Groudon coming up, which Eggy should be solid against.

Eggy is the best grass attacker right now (unless you're really fond of Ho-Oh with Solar Beam and without STAB), and probably will be for a while. Sceptile doesn't look quite that strong, although it's better than Meganium (not that hard to do). Everyone should have at least one strong double grass Eggy in their set of front liners.

Find the best high Iv eggy, turn to SoB if not yet, gradually power up! You will need some grassy attcker anyway! Hard to build all together at once!

Charged TM that CP 1635 Exeggutor first.

Personally I would evolve high level Bellsprouts (hoping for RL/LB) if you don't have enough Exeggutor candies.

Soloing Omastar does not necessarily require a lot of dust as many mediocre grass mons at L30 do the job provided they have double grass movesets with Solar Beam or better. Exeggutor, Victreebel, Venusaur, Vileplume, Tangela, Sunflora all have a clear time below 180 seconds. You can round up the team with Machamp/Meganium/Raikou which are slightly slower than the clock but can still help with finishing the job.

The first Eggy and the Venusaur are good and the Seed Bomb Eggys are kind of ok-ish filler. If you can add two 2k-ish (or higher if Eggy) Solar Beam attackers into the squad you have a chance to succeed. It will be close and will probably require dodging though.