IMO, Gare du Nord and Bruxelles Midi are identical in terms of security, I had never ever a problem in either of them, however, you may see scary people in both places. Same for the RER from CDG to Gare du Nord, I would not object taking it with baggage, especially on a Sunday, however I would not take it during weekday rushhour, not because of security aspects, but it will just be too crowded to feel comfortable with luggage.

You said, you will stay at a CDG hotel, in this case I'd take the TGV from CDG to Bruxelles and change trains there. There are (far) less trains than from Gare du Nord, but if you have a certain amount of flexibility, you certainly find a schedule suiting your needs. This way, you spare yourself transporting your luggage from CDG to Gare du Nord. Bruxelles Midi is quite easy to change trains and has rolling stairs to access the train platforms.