You missed the 350 bus leaving from Gare de l’est passing through porte de la chapelle. Since I moved to the north I’ve taken this bus and it’s a great alternative to the RER. The bonus is that it’s much more chill and immediate. The bus arrives right in front of the terminals of departure/arrival and takes as much time as the RER. Plus, it’s less likely to be on strike or affected by construction work.
Can’t say it’s a scenic ride but it’s relaxing, especially in comparison to going to the Gare du Nord and fighting your way through commuters etc with your bags.
I also think that the price is one ticket T. Someone recently shed some doubt on that so I am less sure now, but I couldn’t find any information on the ratp website specifically.