Is it aiming to get souls for mythicals?

I’ve been really lucky and managed to pull Joan and Siegfrund on my 3 month old account.

I’m really hoping to get some souls for them, but I’m wondering if it would be worth it to use soul coins to buy soul stones for more pulls? Or should I just use them to buy essence and let the souls come when they come from the soul stones I get from events?

Also if you could share how you manage your resources in the alter of souls in general, that would be extremely helpful.

Thanks