A stealth attack should basically be a normal attack except you roll Stealth and the opponent defends with Awareness. You don’t make a “surprise roll” and then a second attack roll after that. It’s much easier to just treat it like any other attack roll vs defense; with extra successes adding to damage and so on. Don’t apply weapon accuracy bonuses to the Stealth Attack roll and you won’t need to add bonuses to the Awareness defense; justify it by saying the weapon accuracy doesn’t matter since if you pull off the Stealth Attack you are going to hit anyway. This cuts down on required rolls and makes it so that Stealth attacks can’t gain such massive success totals that they turn normal attacks into instant kills; remember, the difference of 5 success, a typical DV, is the difference between an attack that might hurt you (3L) and one that may kill you (8L).
It does mean fighty types should invest in Perception + Awareness, but I have no problem with that. If it really offends you, say that stealth attacks automatically fail if the victim or any of his allies sees you, so you have to beat the highest Perception + Awareness of the entire enemy group. Say that so much as a shouted warning forces the attack to revert back to normal combat resolution. This incentivizes the Stealth monster to target the Perception monster first, which I think is just fine. It also disincentives splitting the party, which is also fine.