It depends on your level of General Awareness. Since you have stated that you will study for optional paper also prior to the prelims exam, you stand a good chance to pass the exam, even if you prepare for the GS papers after the preliminary.
Going by the last two years GS mains exam pattern, other than a balanced opinion on current affairs, only a basic understanding of static topics is expected. For example, this year’s mains had asked the factors that caused industrial revolution in England and not in other countries of Europe.
Like I said, if you are a well informed, balanced person, then by all means start after the prelims. However, if you think you fall short, then do preparing now itself.
For example, let’s just look at your English. Please don’t take this as condescending or insulting. My sole intention is to help.
If you are going to chose English as your medium, going by the description of the question you have provided, I see considerable scope for improvement. Remember, you will be up against hundreds of urban candidates who can frame a very good sentence to articulate their views. To match them, you need writing practice which you can’t simply gain in three-four months time.
So, consider your strengths and weaknesses and assess objectively.
It is better to sweat in peace than bleed in war.