I have a buddy that designs, installs and repairs septic systems... he is adamant about how fragile these systems are... SO I don't take chances though I'm just going by what he says.... so I would not drop that tree.
Though the tree is leaning to such a degree that it won't pick up quite as much speed during the fall, I wouldn't count on that effect to preserve the septic system. Its plenty big and there is going to be plenty of mass and momentum to do some damage if it hits wrong.. Observing how trees go through rooves and land on lawns is helpful. Depending on the shape and branching structure a large limb can break and the stub can pierce the ground, so part of the equation is simpl getting lucky or not with where those broken stubs hit. IME they have a tendency to find the irrigation system.
So I would disagree with the suggestion that a wide face and thick hinge will significantly lessen the impact. If you were trying to do that, a block face would be best, but it's still unreliable.
Believe it or not this could be the perfect situation to use an intentional barber chair... It would be safe because it's expected and you have plenty of room for a clear escape path. You could use a back release for the barber chair to make it that much safer. And I GUARENTEE that the top is going to have a softer landing with a BC than with any type of hinge.
1) The top will be slowed down as it needs to break wood along the grain for much longer than a hinge will last.
2) there is some counterbalance to the top by the weight of the back as it lifts
3) the angle that the tips hit does a better job of spreading the weight across a larger portion of the canopy.
I've seen it with white ash many times ... the tree will explode on impact when felled with a hinge but have minimal branch breakage when a BC is used.
The only caution would be if the tree stays on the pedestal and you have difficulty getting it down. IME they will usually roll off when the top is bucked, but that's not always the case. I have a loader with grapple to pull it off if needed, so I never have to worry about how to get it down. If you don't and aren't comfortable with handling a hung BC, then go another route.
so my recommendation would be to plunge in and cut up to leave about 25% of the front wood where the notch would normally be, then come straight out the back and when it starts cracking and popping, get out of there on your escape route, on a 45 degree angle back from the stump. and video tape it!!!
check out this BC on a huge ash and the very little amount of broken tops showing how soft a landing that monster had. If I had dropped that tree on blacktop with any type of hineg it would exploded on impact. That's the nature of ash.
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…or not. 
