You're only allowed to have one of each item across your entire team of six.
Leftovers on corviknight is fine, but make him literally any +defense or +special defense nature (probably special defense) so long a it doesn't negatively affect the other defense. Swap EV's to 252 hp 252 spD. Swap out bulk up for iron defense. From here you would need to swap any other move with body press, and then potentially swap mirror armor for pressure. Corviknight's two weaknesses are fire and electric, but if you roost you trade your electric weakness for a fighting and ground weakness, so you could viably trade roost for a good coverage attack, preferably one with stab. Iron head would probably fill that role fairly well, but I'm not sure if corviknight learns it. If he doesn't then steel wing instead.
Malamar @ Light Clay with careful nature, 252 atk, 252 hp, and 6 spD (survives a full-powered dracometeor from dragapult assuming there was no helping hands or stonejourner buff).
Ability - Contrary
This build for Malamar makes him useful enough to set up screens without prankster or investing into speed EV's on a low speed pokemon. Topsy-Turvy can be used to counter drum-lax and beat-up whimsicott teams. Or it can be used on your own pokemon to reverse the effects of close combat, draco meteor, etc.
From here you'd likely have to redesign your team lol too much would be different. Viable alternatives would include indeedee to replace tsareena. The hidden ability on female indeedee is psychic surge, which would place an anti priority move psychic terrain on the field while also boosting psychic attacks by 30%.
If you do use indeedee as a follow me replacement for tsareena you could respec togekiss for damage, possibly even running super luck as her ability plus a crit boosting item, which according to serebii, would put her at a 100% crit rate on attacks that don't give a +1 to critical hit ratio.
Replace Mr. Mime. Or at least respec his EV's. Running +hp and a focus sash kind of defeats the point of both. If you're worried about getting past screens to do damage then replace him entirely. Any pokemon with infiltrator gets past it naturally, and brick break and psychic fangs destroy barriers as well.