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Well, the good news is that it's trying to make a setpoint, but falling short. You're right to switch to CoolingSetpointModulated and assigning a Heat Exchanger Setpoint Node Name. I would try and autosize everything if you aren't already (especially all of the pumps), making sure your design parameters are set to values sufficient to make the desired setpoint (in particular Sizing:Plant).
Also try using an Ideal HX model type. Although even then you'll probably only be able to get to within 1 or 2C of the supply temp of 6.67C. Autosizing should give you some insight into the loads on the secondary loop though, as they may be such, compared to design values, that the loop just can't meet the operational demand.