how do you unlock more awakened slots?

do you keep on paying 5k gold for it or use plats?

The 5k gold awakening elixir kit is just for the initial awakening.

After that, you need to buy the more expensive awaken GEM kits, such as the ones you craft from incense, or those elite awakening gem kits. Slots will open randomly - sometimes just a few kits, sometimes quite a few.

You can also use a few of the same items to see if you can get a more slots opened using the initial awaken.

You can also have multiples of the same item just to keep the awaken stats you are not sure you want to destroy to get something better.

is this dark legends detail?doesnt seem right

Oops. My bad. Did not notice, but Dark Legends should be something similar, though probably different names.