Another line of investigation could be:
Imagine the DWH heating so the boiler is steaming away at 30kW and there is near-zero flow into the CH zone. When the DWH zone valve closes, if the CH valve is open (or leaking) the 30kW is diverted into the CH pipework which will heat up very fast and clonking is the pipes expanding rather suddenly. The ABV will mitigate but probably not cure this, as observed.
If you go old-school and put a balancing valve in the DWH circuit then you can increase the impedance of that loop slightly so that there is enough flow into the CH branch to keep it warm while the DWH zone valve is open.
You can test this idea by putting your finger on the pipe below the CH zone valve. If my guess is right it's going to be relatively cool while the DHW is heating and get very hot very fast when the DHW zone valve closes and the clonking will start soon after.