Hi London. Welcome to Pprune.
One of the golden rules of aviation, is that 'things always take longer than you think they will'.
With this in mind, you are setting yourself up to fail, or worse, by putting such time preessure on yourself.
As to the 'Pre Flight Checks', well theres a clue in the name!
MJ
Ps. Yes, you are correct to say that 'officail dark' is generally accepted as being 30 min. after sunset.