Welcome to R2R! I’ll +1 to adding some chaeto into the sump to help bring the nitrates and phosphates down. What kind of light did you have over your macros?
As for the shrimp, is there any chance of heavy metals in the water? Even small amounts can be detrimental to inverts... might be worth throwing a poly filter or some carbon in just in case.
And I agree... 6-months in puts you right about in the middle of the “uglies” stage. Keep up with water changes and work on bringing those nutrients down and it will work itself out