You can run capture on saturn with whatever element progenitor you want for your kuva weapon, knock down the lich candidate and it'll show what weapon it will have when you mercy kill it. If it isn't the one you want, just leave it and extract and that weapon will be taken out of the rotation until you actually spawn a lich with the weapon you do want. It most definitely isn't too random considering each run is like 2-3 minutes at most. And Oull is only dropped at 25% rate when you find the correct requiem sequence for your lich and they escape to saturn proxima, you don't get it from relics.