Personally I would consider it a bit risky to do the Islas Ballestas and hope to get back to Lima in time for your 8:00 pm flight, considering you must be at the airport at least two hours prior to departure. The bus ride Paracas Lima will take around three and a half hours. Calculate half an hour from the Lima bus station to the airport. Add a hour for "hiccups". This means you should leave Paracas by at least 1:00 pm.
Several bus companies cover this route throughout the day, but getting timetables online is almost impossible.
Ormeno is an option. There will also be local buses coming from Ica and Pisco. You will have to do your homework on this when you first arrive in Lima. If indeed you can find transportation out of Paracas by 1:00 pm, you should make your flight. Nevertheless always keep in mind that Peru is a country of unpredictables.
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