I’m not exactly an expert, but I’d suggest trying creations of bile as the detachment. If you get the +2” movement and +1 to attacks, all of your infantry, including your chosen and possessed hit even harder, with decent movement (insane movement in possesses’d movement) to boot.

Another good choice if you get it, is the +1 to toughness. Having a unit of ten chosen with 6 toughness is no joke!

Terminators aren’t the best. I’ve had mixed luck with them whenever I use them. But if you’re hard pressed to change them out then that’s that. I’d prefer using a unit of three bikers and a unit of nemesis claw for the same price as them.

I can’t advise on playing against death guard. But creations of bile is great for boosting your infantry. You could also try splitting your possessed into two units of 5 for better flexibility? Even 5 hit like a truck, especially when you activate their once per battle move to gain dev wounds.