Yes, I am concerned about the wood under the hearty. Fire codes such as NFPA 211, or your local building code, IRC, BOCA, CABO, or whatever, all call for there to be no wood at all anywhere beneath the hearth extension. So I would recommend taking out the wood that was under the old hearth, drilling some holes for rebar horizontally out from the existing chimney where the hearth used to be, in the floor space, putting a steel form plate across the bottom of the framed opening thru the floor, and pouring about 6 inches of concrete there. Get the rebars in it to support its weight on the chimney, not the floor.. Then build a masonry hearth. It could be lightweight masonry, hollow blocks taking most of the space, and a nice facing like you have now. The other glaring problem is the wood firring on the face of the fireplace. There is not supposed to be any framing wood, or wood at all except trim, in contact with the chimney. Everything looked pretty good when you tore it out the first time. I couldn't tell when you put it back if you had maintained proper clearances, but improper clearances there are a major cause of structural fires. I hope this is helpful. I'm all for heating with wood, SAFELY. My house burned down from a faulty installation when I was 8 yrs old. I don't want to see that happen to anyone!