you can just buy all runes and farm 1 spark, to transform into desired uber unique?

I believe you need to farm 2x sparks and you can craft a random mythic

If you acquire (farm/trade) “runes” you can use those with 2x sparks and craft a mythic of your choice

I have no idea what runes or the quantities needed to target craft your mythic. Not sure if that info has been released yet (might need to wait until the 4th when the streamers/testers can share the info)

You need 10x magical, rare and legendary runes of the same kind + 1x spark to create a mythical item. For example, if you have Jah as your legendary rune, you need 10x Jah for the recipe. Good luck farming all of that

4?

Only 2x Sparks after the expansion hits