The 7-segment LED display is too dark when connect to a auto dimming circuit
Connect a 7 segment driver to the output of the circuit. That IC will manage all current calculations. IC ULN2003 seems to be a good option for your need. However, if you're not able to find a suitable driver for your need to design a circuit using MOSFET and connect them parallel to the output of your circuit. Since Mosfet is Voltage controlled device, you can connect many MOSFET in parallel, they will all run the same intensity.