The Sona nerfs are over the top and need to be scaled back

Let's address the first big issue. This change does not really effect late game Sona. You know the point in the game where Sona actually gets good. It hurts early game Sona, the point in the game where she is weakest and honestly needed buffs if anything. Before this change Sona had a damage output of 60 damage + 80% scaling (assuming you landed both parts of your spell) for a cost of 45 mana. After the change she now has an output of 50 + 90% scaling (assuming both parts hit) for a cost of 60 mana. Since Sona is not going to have a metric ton of AP early, the scaling is more or less unimportant. As it stands Sona now has a below 1:1 ratio of mana to damage dealt (i think the only champion in the game with such a metric).

Sona's early game has always been bad. Your damage output of spells is low, and only looks good because more half decent Sona players are landing AA + Q +AA reset + Passive. An issue that could be fixed if Riot simply removed auto attack resets... but that would ruin the "unique play styles" of so many other champions. Your healing is expensive and does very very little to help your allies or yourself. You have no way to engage or disengage fights, and your movement speed ability is garbage. If we look late game, you notice that Sona is now doing a wopping 20< less damage. She still hits you like a truck, because she is still hitting you with a 5 in 1 part attack.

There are so many easy ways to deal with this issue that aren't ham fisted and crippling for Sona's early game. Something as simple as bumping the casting cost up by 5, rather then 15, and having it go up another 5 at rank 3 and finish at +15 at rank 9. BOOM keeps the late game mana pressure but doesn't make you spend 20%+ of your pool every time you poke in the early game.

If you want to decrease Sona's early game poke, that is fine. You want to reduce her late game damage, you need to give her something else she can do that isn't being a one and done ultimate caster. But over all you need to NOT make her already weak early game (do to bad utility numbers and mana constraints) worse by upping the casting cost of a spell she is EXPECTED to cast 3-5 times a minute by 30%+