Use a primary secondary pump setup. Run a 28mm "primary" loop from the flow of the boiler to the return. Put the large pump on the flow just after the boiler. At roughly the midway point of this loop insert two tees next to each other, as close as possible. The tee nearest the flow to the flow to the CH/DHW and has the smaller existing pump on it, or better still replace with a Grundfoss Alpha auto variable speed depending of system pressure, so TRV can be on all rads, and the associated zone valves. The second tee is the return from the DHW/CH.
This ensures full flow through the boiler at all times. Set the large pump on the "primary" loop to flow required, not too much, just keep it the right flow.