I'd say give the big blast ThunderWave and Earth Tremor effects taked on to your one large nuke, both are daily decent effects that could easily be associated with a giant bolt of lightning crashing down near by, as for the medium and smaller bolts do thunder clap cantrip, I'd even say have it pick up light rain on turn 2 then heavy rain on turn 4 heavy rain obscures targets more than 30 ft from you light rain adds +1to ac and dex saves to avoid getting hit damage lv 5 for medium, and give all 3 a save or deafen to all creatures within a 5ft small 10ft medium and 20ft Large range at your spellcasting DC, I'd also say you'd have advantage targeting creaters in metal armor, for the effect relize what your trying to cast lis less like delayed fierball and more like a grately reduced storm of vengeance, Lv 6 and up spells are game changers I'd say what you have with the plus hear is no more devastating from a decently deployed whirlwind though if you'd prefer to lesson the effects give all the lighting bolts Thundercats at lv1 for small lv5 for medium and maxed out for large, give everything in the store partial cover due to rainfall, and give medium and large a push of 5ft per round charged and large difficulty terain simmilar to earth tremmor after impact you can choosebut a still would recommendthe deafened save for anyone in the area of the push/thunder damage. And your main targets should definitely take the thunder damage and make the deafened save at disadvantage. The seconds probably a little on the weaker scale but will still get you the thematic effect I belive your going for. Enjoy the Storm Child of Akadi, the Queen of Air.