I would try out a Paladin/ Sorcerer build, if you are comfortable with multiclassing.
Conquest Paladin would be best, but is not in the game. So I would go with vengeance.
In 5e, you do not necessarily have to get your powers from a god anymore (though you can of course be a devout follower of Tiamat, that would certainly be a way to see the world in flames). Your oath could basically be to kill everyone who tries to bring harm to a dragon or resist your conquest of inferior beings.
With the sorcerer levels, you get better spell casting, including aoe damage spells, as well as more slots for smites. As well as metamagic, which is also very strong in bg3, since there are no longer restrictions on the number of spells every turn, if you are hasted, or use quicken...
I would go with Paladin 6/ Sorcerer 6, personally, but there are different ways to optimize this. With that, you will have extra attack and smites, be a full martial, have aura of protection etc, as well as 5th level spells. So you only lose lv 6 spells compared to full spell casters
Edit:
Of course, choose Draconic Sorcerer
It is not necessarily optimal, but it is your blood heritage, and the source of your great power, in addition to what Tiamat has given to her champion (or what you believe she has)
