The only thing i fear is that higher level enemies will be prepared for such kind of damage. :-/
So speaking purely for myself ... as usualy ... as long as the game will be beatable without using such "tactics" (for lack of better therm) ... as it is now, in EA ... fine by me.
I bet that Sorcerer player will feel incredibly clever first time he uses it.

Essentially this. Either encounters will be balanced assuming players use these OP strategies, in which case non-caster (particularly non-sorcerer) players will feel ineffective and underpowered. Or encounters aren't balanced for these strategies, in which case these strategies trivialize combat (and again, make non-caster players feel ineffective and underpowered).
A key point is that BG3 is a multi-player game, so balance between players and classes are important to game-feel. One class/player dominating combat
can detract from other players' enjoyment.
It is better than RAW but still not great to me. Having Quicken be a sorcerer mechanic that can’t be used for full-action spells does not seem good to me. I wouldn’t mind if a few spells, such as fireball, were excluded from Quicken but having all spells excluded is too much for me. I like how BG3 does it much better!
It wouldn't make sense to arbitrarily restrict a select number of spells from being able to be Quickened, and would likely frustrate and confuse a lot of players. It takes the same amount of effort/time/magical energy to cast fireball as it does any action-cost 3rd level spell.
IF Quicken in BG3 is going to remain as it is, then its sorcery point cost
needs to scale with spell level. A constant cost makes sense in 5e RAW because, essentially, Quicken only allows you to cast an extra cantrip or take basic non-magic actions (Dash, Hide, Dodge, etc) no matter what level of spell you quicken -> you're essentially quickening those actions. But in BG3, your turn becomes significantly more powerful with the higher level of spell you Quicken. E.g., quickened magic missile + fireball vs quickened fireball + fireball. Thus it should cost more sorcery points.
Edit: To tie this to Circlet of Fire - the extra bonus action given by it is much less powerful if you can't exploit Quicken this way.