When I make a bigger porteur bag I'll usually sew in a couple of straps that I can loop around the handlebars to keep the bag from sliding forward (I use drop bars on everything I ride, so I can't just have huge loads expand sideways because then they infringe on my shifters and brakes) but the loads I'd put in those bags are enough to bend an integrated decaleur all to hell and back.
I'm not sure about the weight gains, though. A custom Herse-style receiver that mounts to the handlebar clamp bolts might weight slightly more than an integrated receiver, but that's because it will need a couple of pieces of steel plate for the clamp bolt mount tabs and those are only 8-10 grams (there is likely less tubing to go from the stem to the receiver that it takes to reach up from the rack deck, but .028 5/16ths tubing doesn't weigh that much so you don't win there.)
Wider racks are better; if the rack is as wide as the bag it won't want to slosh off to one side or another as enthusiastically as it would on a skinny rack. If you've got a tall unlined bag with any sort of load it will shimmy nevertheless, but if the load isn't heavy enough to make the bag sag forward you don't need a decaleur and if you've got a cargo net you can hook it around your handlebars & the rack to keep everything in place.
(I've only got one rack that's narrower than the bag I put on it -- a 7x7 rack on my light bicycle with a 9x6x7 rando bag I originally sewed for an 8x8 platform. It shimmies more than the better-fitted racks and bags, but thankfully not enough to be unridable.)
-david parsons