Heavy traffic is unusual anytime in August. On a Sunday morning in August, there will be no traffic.
Uber is not necessarily cheaper than a taxi. On my last trip to CDG a few weeks ago, the RER B was unexpectedly slowed due to a power problem. Uber quoted me 100€. I paid a taxi driver 50€.
The way in which you have constructed your question, I infer that you want to save cost and use your Navigo cards. RER E/B is a simple solution with no additional expense.
No matter the choice, allow one hour transportation time. Reliability with any option, is always a concern. Remain flexible and have alternatives at the ready.