How to improve this amplifier so I can get up to 5 Watt output in 8ohm load
Just looking at this simply, you have a "Darlington" output stage and this inevitably "loses" about 1.4 volts when powering a load - basically, from a 12 V supply, you can't deliver a "high" voltage greater than maybe 10.6 volts. To avoid distortion maybe 10 V is the maximum.

So, 10 V is the range of the signal that can be produced. In other words the output maximum level is ~10 Vp-p. This is the same as an RMS voltage of about 3.54 volts. If this feeds an 8 ohm load then the power delivered is about 1.56 watts.

That's the limit from 12 V I'm sorry to say. If you want to know why your amplifier only gives 0.8 watts you have to tell me more about the bias points in the circuit and how much distortion you are tolerating when 0.8 watts is being produced.

Clearly you could produce an output that is a square wave of 10.6 Vp-p and, into 8 ohms this is a power of about 3.5 watts but I suspect you are doing a sinewave measurement.