Not sure why the Gunnar Hyper-XX doesn't get more love... maybe lack of a marketing budget?
Changing wheelsets (and tires) will give a ton of flexibility and make for a solid "do anything" bike. It will take a 29er wheel with a 2.0 tire if you're riding really loose stuff, but 700c with +/- 40mm seems to hit a good spot for reducing weight to take on hills/accelerations and float (and I know they're the same diameter, just sayin').
Steel frames are never as light as carbon or alloy or Ti but a 21-pound build is not hard on this frame. Even going with 105/5800 Hydro should still keep it in the lower 20s.
I'd personally look at a Whisky fork. I've got several buddies riding them hard with good results. The headtube will take a tapered headset. If you reduce axle-crown you lose some of the choice for a larger front tire. You might have the width but not the height under the crown.
You'll prolly get a different BB drop depending on tires/wheels. Not sure I'd overthink it. I've had bikes from 67 to 80mm drop. Lower gives more stability but unless you're regularly descending at 45+ mph I don't know think you'll have a problem at 70.
Would be great to hear what you decided, or if you went in a different direction.