RER D has a weird weekend timetable, with some annoying gaps of up to 20 minutes in the central section (followed by a train 3 minutes later).
The good news is that, if this is the case, you can get RER B from the adjacent platform one stop to Chatelet Les Halles, from where it's a cross platform change to RER A one stop to Gare de Lyon. These lines are very frequent.
I count Pta Garibaldi as being in central Milan, I suppose it's a little on the periphery. Never felt unsafe there in my few out of hours visits v- it's certainly no Brussels-Midi, which has to be one of the least savoury major mainline stations I know!