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It may be possible your pump is not connected / integrated with the boiler system such that it is setup in a fixed way. You may want to investigate the pump configurations available if they are separate to the boiler controlling/modulating it.
The other thing is that as you’re already running really low flow temp, then return maybe can’t actually drop that low as the flow rate would be just too slow.
Also, your 22mm primary pipework is good for 18-24kW at a dT of 15-20C on a gas boiler:
For a heat pump, yes an optimum target of ΔT5 is approx ~6kW on 22mm pipework.