huge plume of brown smoke coming out of the exhaust
Brown smoke = too rich. So that’s showing an overfuelling problem – at least that’s the visible symptom.
If the coolant is ok, that probably eliminates the head gasket, generally with a bust head gasket you’d see problems with coolant.
The horrific oil consumption would suggest the lifters aren’t to blame for the fuelling problem, broke lifters wouldn’t affect the oil.
I’d agree with TR on piston rings.
Whatever it is, its most likely a stripdown & rebuild job – not a cheap repair.
Plus by now, cat converter and possibly O2 sensor will be shagged. (edit – *maybe* – you might have been getting away with it if the fuel mix isn’t too rich for the cat/sensor)
Best of luck.