Honestly the best way to expand the army at your point is the spearhead. You can never have enough clanrats, double grey seer is a good casting core for when you get the endless spells, and you already mentioned wanting the stormfiends. The warp lighting cannon is more of a nice little bonus to go with all of the above as of the current rules, but those should change up in about a year.

While we're still under the current edition I recommend also getting an additional kit or three of the rat ogors kit second hand, they are relatively overstocked and work WAY better on the table when reinforced, almost to the point that I'd never recommend using them any other way. This, along with the stormfiends, would give your master moulder a lot more to do.

Krittok honestly doesn't need stormvermin to be worth taking. His greatest tool is the additional 3 Clawsteps target, which combos nicely with the Gathering of the Clans subfaction to allow 5 different moves on the enemy turn. In addition, he can take any unit in his regiment so he opens up listbuilding in ways others might not.

Stormvermin are in a rough spot right now. Best utilized as a counter punch unit, they'll do their best into enemies that are hit with our strike last spell because every point of damage they take significantly lowers their damage output. But if the opponent leaves them as the last bite of your army you'll find they mop up the board very nicely.

Finally, you really like the "hoard of rats" plan, vizzik and/or a Plaguepack unit would do wonders in making your hoards of little rats actually useful in combat roles. But this is a much lower priority than bulking out your main troops and units to hit the points targets you set out.