Edit: teams 1/ 3(both not necesarry for fire start, but atleast they are the same element, need some investment)/ 4 gaia and Odin are a good start for wind/ 5 /8 are viable options.

If you go with dark and want to improve it through miracle tickets (longterm) you should get Satan imo, she is not strong right now, but after awakening in august she will be a beast(if you want to use the hades account) or get Hades if you want to use the amon unleashed account. If we are lucky we can actually get Thanatos with the next ticket (not sure yet how nutaku will handle this next ticket).
Heal is rarely a problem unless its very long fights like with accesorry.

Souls you should get d'art/mordred/joan are the mandatory ones. if you have serious issues cause of a lack of heal, you can opt for andromeda afterwards.

Wether you want to reroll for thanatos or not is your call, but then you would still need a combination of hades +amon/satan.

but wait for some more advices or take a look at the elemental build guide.