How should I find the DC bias values for this circuit?
I'm not sure if I can assume Vout and Vin+ and Vin- to be 0 at the
quiescent point also. How then should I proceed in the calculation?

You pretty much can assume this because the DC gain of the circuit will be in the realm of maybe 1000 or even higher and, the effect of negative feedback via RF2 will ensure that the mid-point quiescent voltage on the output will be within a whisker of the ground voltage set by the lower end of RIN1.

How then should I proceed in the calculation?

By assuming what I said above is true.
BTW I would look to making most of the 11 mA current consumption in the quiescent condition pass through Q11 and Q12 to avoid unnecessary crossover distortion. If that means 10 mA for Q11 and Q12 and 1 mA for the rest of the circuit then so be it.