A 5th level spellcaster only has two 3rd level slots. No matter how you slice if dice it they could only do it once
Yep, that is what I said "both 3rd level spells". And at 5th level it is hugely significant. They only have 5 points. They can only do it one turn whereas if they used it normally they can use it in two turns.
And, 3 sorcery points isn't materially different enough from 2 sorcery points to justify allowing two fireballs on the same turn. That's only 1 extra sp!
Anyone would pay 1 sp to be able to double fireball. On the scales of cost v benefit, that's ALL benefit.
Sure but they can only cast it as a bonus action once this way at 5th level as they only have 5 points. They could cast it twice the other way. If they get to level 12 and have that many Sorcery Points who cast normal 3rd level spells when you can upcast it? Say they do use their points to only cast Fireball at 3rd level, they can do that only one turn before having to use higher level slots which would then cost more Sorcery Points. First round is 2two 3rd level spells and 3 sorcery points (9 points left 1 3rd, 3 4th, 2 5th, 1 6th). Second round is one 4th, last 3rd and 3 more points (6 points 0 3rd, 2 4th, 2 5th, 1 6th). Third round is one 5th, one 4th and 4 points (2 points 0 3rd, 1 4th, 1 5th, 1 6th). Fourth round is use 5th level spell to get 7 points (8 now) use one 5th, one 4th level and 4 points (4 points 0 3rd, 1 4th, 0 5th, 1 6th). Fifth round is 6th level, 4th level and 4 points (0 points, 0 3rd, 0 4th, 0 5th, 0 6th). Now they are completely taped after 5 rounds using fire spells at level 12 where almost all monster have resistance to. Still an amazing set of turns that most likely have ended quite a few enemies non the less. The problem is that was only one encounter, what happens during the other 7 during that adventuring day. Say the used this technique only once per encounter they could only do that in 6 encounter and only for one round of nova without using anything higher then second level the rest of the turns.
Another point to this being a silly rule is persistent spells like Call Lightning, Flame Sphere, and Witch Bolt (less on Witch Bolt as the persistent damage is poop). You cast Call Lightning at 4th level and can now use your action to basically cast the spell again every turn then use Quicken Spell to cast Fireball as a bonus action. Using Call Lightning is not castings it on subsequent turns. Those have way more potential for damage then a one off Fireball that the monster most likely has resistance to and will save from the damage and that is doable RAW.
I think give more proof that the spellcasting bonus action rules are not right and should be removed for DnD 6e. You can already cast a leveled spell as a bonus action and basically cast another leveled spell as an action. Having Call Lightning up allows a Sorcerer to Metamagic Quicken Spell to cast Fireball. But even no Sorcerer you can have Flaming Sphere up and cast Fireball as your action then move Flaming Sphere to hit multiple enemies doing the same damage as if you'd just cast it. The bonus action spellcasting rules make no sense.
For the Quicken Spell shenanigans you could have it cost the same as Twin Spell, 1 sorcery point per level of the spell(minimum 2). Two fireballs are much more expensive now. They have now used 3 of their 5 sorcery points both 3rd levels spells (should they only be 5th level). One turn of nova then their taped pretty much.
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Another point to this being a silly rule is persistent spells like Call Lightning, Flame Sphere, and Witch Bolt (less on Witch Bolt as the persistent damage is poop). You cast Call Lightning at 4th level and can now use your action to basically cast the spell again every turn then use Quicken Spell to cast Fireball as a bonus action. Using Call Lightning is not castings it on subsequent turns. Those have way more potential for damage then a one off Fireball that the monster most likely has resistance to and will save from the damage and that is doable RAW.
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I think give more proof that the spellcasting bonus action rules are not right and should be removed for DnD 6e. You can already cast a leveled spell as a bonus action and basically cast another leveled spell as an action. Having Call Lightning up allows a Sorcerer to Metamagic Quicken Spell to cast Fireball. But even no Sorcerer you can have Flaming Sphere up and cast Fireball as your action then move Flaming Sphere to hit multiple enemies doing the same damage as if you'd just cast it. The bonus action spellcasting rules make no sense.
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For the Quicken Spell shenanigans you could have it cost the same as Twin Spell, 1 sorcery point per level of the spell(minimum 2). Two fireballs are much more expensive now. They have now used 3 of their 5 sorcery points both 3rd levels spells (should they only be 5th level). One turn of nova then their taped pretty much.