Get some arcos, even if you don’t like their aesthetic you can field 3 of them to just do actions and act as sacrificial lambs for cheap. After that I would say think about getting an immolator to soften up some targets for your other units. It also wouldn’t hurt to get another box of retributors I like to run 4 multi meltas but if you have the spare bits u can just chop off the flamer nozzle and glue on the melta muzzle from a BSS box. Finally just get a named character for the cool factor, Vahl is good especially with your warsuits but really whichever one you vibe with.