You could always go for a backstory along the lines of "I was offered up to Talos as an infant, but now I travel the lands trying to mitigate the harm storms do." Or something of that sort. The one step alignment rule isn't part of 5e, and deities are no longer able to remove spells. Given how BG3 tends to work, the will probably be a few {Talos] dialogue options, but you don't have to choose them.
Or, you can interpret the domains more flexibly. In earlier editions Silvanus* used to get elemental domains, which are not in 5e. So Tempest = Elemental Air.
*Is Silvanus on the BG3 cleric list?
