item: two items that related to the fire damage. the headband from rescuing the Counsellor in Waukeen's Rest. and a pair of gloves for rescuing Baelen in the underdark.
While bonus action is fine ... i dont find a lot of uses for it on Warlock, to be honest ...
Personaly i prefer Circlet of Blasting from Blurg in the underdark ... it does no initial bonus, but allows you to cast one free Scorching Ray, wich i personaly find quite usefull, since Warlock have so limited amount of spellslots.
https://baldursgate3.wiki.fextralife.com/Circlet+of+BlastingThere is also magic ring, that will add +1 to every fire damage you do ...
https://baldursgate3.wiki.fextralife.com/Ring+of+Fire ... it isnt much, but there isnt many better options anyway.
And for maximum effectivity i would also recomend equipping Shadowheart with this:
https://baldursgate3.wiki.fextralife.com/Dragon's+Grasp//Edit:
And since (as i said) i dont see usualy much uses for Bonus Action on Warlock (read as: When i dont presume i will need to run next round)
I also recomend using:
https://baldursgate3.wiki.fextralife.com/Ritual+Dagger ... especialy for Wyll, since his spell-hit chance is a joke.
