The orginal brackets/elastic from RS can be a little loose, adding some more tighter strapping behind the ab/chest should help.
Notice the strapping in this image
I like to use adjusters between the chest and ab so it can be adjusted if needed.
Looks like the chest to backplate are held as far as they can go, you could remove the shoulder straps (depending on what glue was used) and bring them further backwards. The ridges/tabs on the front can vary depending on armor so only having 2 or 3 smaller tab with the larger tab should be enough.
Rough image but you should get the idea, you may need to bend the front straps slightly, some use a hot water bath, I use a heat gun but I've had a lot of practice
You are pretty stuck with the positions of the rest of the armor on you, very hard to make it extend unless you add loose strapping but then you will have gaps between the armor pieces.
I would adjust the chest/ab first and see how everything is looking then