On my return trip home from Gare du Lyon to CDG (08/21), I was hoping to use only the metro (RER A followed by RER B) i.e. the opposite of my arrival (RER B and RER A) on 08/16.
Why is this option not available on the SNCF website?
The RER A and RER B train lines should be operating normally on August 20, 2023 and you should be able to take the RER A + RER B from Paris to CDG Airport without issue in late August. (No rail works are planned for August 20, 2023 for the RER B, RER A along the route from Paris to CDG airport).
Here's a sample itinerary for Paris (Gare de Lyon) to CDG on RER A + RER B on August 20, 2023.
Btw, someone else was asking about itinerary searches not showing RER train results for August 20, 2023. There's perhaps a technical issue/problem with the itinerary builder from RATP.fr for this date, but Transilien which handles RER trains outside of central Paris, is showing RER results OK for this date.