According to the circuit diagram you gave, the simplest way to increase the transmit power is to reduce the resistance of R4 and increase the base bias to increase the transmit power.

In addition, adding a level of pre-amplification to the MIC signal can also increase the transmission distance.

The principle of these two methods to increase the transmission power is to increase the excitation of the signal, which can be understood as increasing the input signal amplitude within the dynamic range of the circuit to increase the output power. Just like the pure power amplifier in audio equipment is equipped with a preamplifier, the volume is controlled by the preamplifier, and the reason why the postamplification remains constant is the same.