Mountain Trader, just for reference it's a break, not a brique. (Yes, English invades the French language once again!)
And I agree with you that I wouldn't want to transfer from the RER to metro with that amount of luggage, either. Just the thought of lugging it up a ton of stairs... no thanks. I would probably go with your second suggestion (Gare de Lyon plus taxi), but then again it's the station I'm most familiar with.
Otherwise, taking RER + metro, I'd take the RER to Gare de Lyon, then line 14 to Chatelet. At least for the GdL to line 14 transfer, the chances of having working escalators or elevators are good--not sure what it's like on the Chatelet end. Although at the Gare de Lyon, if memory serves, it's a heck of a long walk to transfer from the RER to metro, especially with lots of bags.