Light detector for row of LEDs
Without validating the circuit provided in the question, here is a minor addition that should address the brightness issue partly (see note at end).

A MOSFET is switched by the same operating point in the original schematic, as was switching the single LED on or off. This removes the dependency of LED brightness on the emitter current of the BJT Q2.







simulate this circuit &ndash; Schematic created using CircuitLab

The gain and thus switching sharpness of the pair of BJTs has not been compromised from the original, partly because I was lazy. BJT Q2 could very well be replaced by the MOSFET directly, if so desired. 



Note:

What this does not solve, is the issue around the current drive capability of the power source. If the 6 Volt supply in the design is not able to supply the current demands of the multitude of LEDs, that will simply require the supply source to be changed.