Oathbreaker/Sorcerer - I like that idea. Dragonborn are "Very Naughty Bois" by default ... As to your char design plans - our suggestions are based upon EA patch 9 plus what is known about 1.0. Come 3.VIII there could be changes to the game we are not aware yet, hence leave margin for "I may have to tweak it" .
Besides distance spells are various "cone" or "touch" spells - like Shocking Grasp - to be used in melee.
With Oathbreaker, I would suggest going Paladin 8/ Sorcerer 4 instead, since Aura of Hate is very strong. I would also pick up Polearm Master (depending on the weapons in the game) instead of GWM.
If you want, make Gale a necromancer, since the skeletons also benefit from the aura of hate. Maybe he is a Tiamat cultist too
If you want to refresh your Tiamat lore to roleplay properly, this video was good, I thought
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