In most of these cases, a proper match design is enough. If the enemies aren't grouped, the fireball will do nothing.
There is no chance that the fireball will retain a 6-meter area of effect when the range is heavily scaled down.
I bet it will be 3 meters maximum in the game, which makes fireball spam much less effective.
There's no reason to restrict this to fireball. Single/individual target damaging spells (scorching ray, blight), single(upcasting to multi)-target control spells (Blindness/Deafness, Hold Monster), AoE spells with different areas (Lightning bolt, Cone of Cold), all of these are quicken-able and can be used instead of/in addition to fireball. They won't do as much damage as fireball+fireball will against a cluster of enemies, but they'll still have a massive effect on enemies.
I can't comment on the size of fireball in BG3. One the one hand, many spells' ranges have been significantly decreased. On the other, Larian likes surfaces and fireball has the potential to create a big surface.
