I will try to build it to a stripboard as you said. Until then, I uploaded the schematics. Please inform me about the problems that you see on it.
In a circuit I once done, instead of your mosfet (Q1), I used a 4066 cmos analog switch, the enable control input tied to the sync separator, and a trim-pot at 1 input, adjusting the voltage from 0-5v, and the other side tied to the video source after a 1uf cap, or 0.1uf cap, not 100uf (C3). (Not if you want a true black level adjuster, tie the 4066's enable to the 'burst' output on the sync separator)
At that video point, I used a 2n7000 mosfet in a source-follower configured amp. (video after 1uf cap tied directly to Gate of mosfet, 1k pulldown on Source which was video out) (No Q2/Q3/R3) I was then able to adjust the voltage output of the video to any level using the trimpot.