Short answer is:
4 Piece Tier Set
Tiny Abom
Death’s Choice or even Greatness
Swap Ashen Ring to Agil Version
Recovered Scarlet Onslaught would have been better than Ocean Serpent
Gem full strength
Reasoning:
So many dead secondary stats. Way over hit cap. All that excess hit, like 400 may as well be spirit. Missing the equivalent of 5+% crit and/or 10% haste
ARP is a Minimal dps diff for you, like 200-300 dps
Your trinkets are almost uneqipped slots in worst cae scenario, -32 Ilvl in best scenario. Mark doing 0 passive dmg, active depends on how you are using it. Herkumi performing at ilvl 232 with slow ramp.
No tier set and maybe some talent tree errors also accounts for anywhere between 500 dps-1000dps
Raid comp also makes a diff, kings might, melee haste, 10% AP all have significant effects on dps performance,
Also, no info about your rotation, divine storm capitalizations, etc