Persist (since it has a fixed range) and extend it. All day long you can throw fire (at least 1,440 attacks worth).
Yes, this is putting way too much effort into the spell when you could do better things with it. However, if optomizing it, better mention these.
Searing Spell is a must for dealing with resistance/immunity.
Let's see, Arcane Thesis (Produce Flame) is probably necessary for any build like this.
Blistering Spell makes it 1d6+7 (no save on Produce Flame, so unknown if the minor debuff takes effect) (with Arcane Thesis, +0)
Energy Adimixture makes it 2d6+14 (with Arcane Thesis, +3)
Empower makes this 3d6+21 (with Arcane Thesis, +1)
Searing Spell makes it ignore resistance and deal half damage to immune (with Arcane Thesis, +0)
Maximise is too expensive for the benefit, really.
Extend Spell gives you double the attacks with the spell (with Arcane Thesis, +0)
This is actually a case where Heighten Spell increases the damage dealt, because Blistering Spell works off of spell level on how much benefitit gives.
Cheap trick: Choose Energy Substitution (Fire) for the prerequisite for Energy Adimixture and apply it to the spell. With Arcane Thesis, that's pretty much a free -1 spell slot adjustment.