Visibility is the big thing. The majority of break and takes I hear about involve a vehicle with an item of value left where folks can see it.
Toppers help minimize, but don’t eliminate this. If you want to go the extra mile I’d keep everything you don’t want exposed in a Decked system. Job boxes are heavy and not super convenient, but obviously more secure in that folks will have to work a lot harder to get in.
If you can lock your tailgate, the Decked systems are fairly secure. I’d feel comfortable (relatively) leaving a firearm locked up like this.
Goes without saying, but never leave gear on your seats.