For what you're doing the back plate and wing is the only way to go. Use a simple Hog one piece harness, maybe a crotch strap to give the rig a little more stability especially if you get inverted (I'm a huge spearfisherman also). I know how all the rules of "proper trim" go out the window when wrestling a big fish and trying to dispatch it, a crotch strap will keep the rig from swinging around.
Forget about chest straps and integrated weights - big waste of time.
Use a weightbelt.
All you'll need for lift is probably an 18 lb. wing, at most a 30 lb., but if you're properly weighted I don't see how you could use all 30 lbs where you'll be diving.
The other nice thing about BP/W is if any one component wears out you just fix or replace that one component, not the whole unit, and you can switch wings for whatever water temp you want to dive.
The actual metal plate will never wear out, your kids and their kids and their kids will be using it for hundreds of years (if diving is still around).
Once you go BP/W you'll never go back, there is no better place to go!
