Quick follow-up for anyone who may be interested:
Train arrived about 30 mins late at Montparnasse, around 8pm. 15 or so people in the taxi queue but no taxis to be seen.so I skipped it and went to LBD instead.
Around the same number of pax waiting for LBD, so joined the group (hardly a queue) and the bus "crew" soon arrived. Bus left 20% full at around 8.20, and just a few pax picked up at Gare de Lyon.
FB Gold card is only valid for transfers between CDG and ORY, not to/from CDG and other stops.
I'd say light traffic thru Paris, arrived CDG T1 about 9.30, T2C around 10 mins laters.
for the time being now my backup plan to M6/RER B between CDG <-> Montparnasse.