"Only time I drove it was 5am on a sunday morning - quite deliberate."

Well planned, my friend. You don't want any part of the 405 if you're actually planning on being somewhere at a specific time.

Real question - how long would it take me to drive south from the airport to Oceanside at about 7pm on a thursday evening?

To quote the inimitable Rene N... "Hahahahahahahahahahahahahahahahahaha"

Actually, you should plan on hitting rush hour traffic... could be two or three hours, easy. Make sure you take the toll road through Orange County... well worth the price.