Data collected by HeatpumpMonitor.org shows that that properties of a similar age and size have consumed between 400 and 800 kWh of electricity in December, so the figures you’ve shown do not look particularly high.
The COP figure is on the low side – we’d expect to see it closer to 4.0 in that month – but the temperature measurement can be off sometimes. You can check if the sensors are matched by finding a moment when the compressor is off but the pumps are running, as André describes here: Why doesn't a low flow temp translate into a decent COP - #27 by Andre_K. There follows a calculation to work out the true COP.
Heat pump efficiency could be improved if it ran at a lower flow temperature, say 40°. Converting to open loop should help with that.
There’s a chance that the heat pump is slightly oversized for the property (surprisingly common), with actual heat loss closer to 4 or 5 kW. This can cause more cycling when the weather is mild, particularly if not all zones are open. This is quite normal for heat pumps though, and up to 3 times per hour is deemed acceptable by manufacturers.
Anyway, it sounds like you’re already making steps to improve the system.