We will be arriving at 10:30 local time at CDG the 1st Sunday of October.. We need to get to Gare St Lazare for a 2:00 train to Rouen. That gives us 3.5 hrs to get through immigration, and take a taxi or Roissy bus (I'd iike to avoid the metro since there is not direct train to St. Lazare) to St Lazare. Will this be enough time? Should I book a later train to Rouen? Will the underground be worth the hassle because it's faster?
The usual recommendation is four hours but on a typical Sunday, 3.5 hours should be enough time. Maybe a local cal let you know if October 1 should be a typical Sunday.
Sunday morning is probably when you'd hit the least traffic, so 3.5 hours feels like enough! With 2 people, I suggest a taxi, because Roissybus for 2 is 32€ vs 55€ for a taxi: the extra comfort of a taxi are probably worth the extra 23€.
The RER/ underground will not be worth the hassle.
There is no traffic early Sunday. You might be able to reach St Lazare by taxi from CDG in little more than 30 minutes. As long as your CDG arrival is as planned, 3+30 is sufficient time to make a Paris train connection.