@liamvhogan
Dunno you can get lurid soft panniers.
If you do get softies then hunt for reflective strips. SOLAS level is white which isn't great when rear facing but it is Very Reflective. You can sew them on to the panniers. Look for them in ship chandlers.
Hard topboxes take up a bit of room but can hold a lot, are lockable, and tend to come with reflectors.
Generally trying to be seen by being bright coloured is more or less trying to interrupt the driver. Which doesn't often work. Better to ride so if they don't see you they won't hit you meaning buffer zones and where you place yourself in the lane.
To quote a firey of my acquaintance: "Headlights on? They don't see you if you are 16 tons, bright red, and howling like a banshee, never mind headlights on"
If you do want to use a backpack and are in Sydney I have one that has heaps of flouro yellow and is looking for a home.