Which type of short cycling? Is the Nest giving an error or does it stay orange during the whole time? Is there any flash code at the furnace? What are power numbers? https://support.google.com/googlenest/answer/9241211?hl=en#zippy=%2Cpower

Sometimes the only solution is to install an isolation relay.

It looks like you did everything right. No surprises there.

I assume that when you remove one of the wires going to the boiler it does shut down.

The Nest Power Connector is an electronic switch so the boiler may be too sensitive for it. You can try swapping the two white wires going to the boiler thermostat connections as that has been known to help with other Nest products (haven't seen with power connector, though). Otherwise you may need to add an isolation relay.

I did find a note from IBC for ‘GHOST’ CALL FOR HEAT - Triac or ‘Power-robbing’ thermostat sending current to boiler. -

Remove Therm. connections from boiler to confirm that stray voltage, or current induced in thermostat wiring, is source of nuisance signal. Replace the Power Robbing thermostat, isolate the thermostat with a relay or install a properly sized resistor (consult the thermostat manufacturer first then IBC for instructions).

Probably but not guaranteed. Sometimes a C wire doesn't fix things and you need a resistor installed between W and C at the hvac end. There is a small group of problem installs that need an isolation relay to solve things.

Their note stated a resistor would work. but that 51 volts may throw things off.

Other thing they said wold work is a relay. An easy one to wire is https://www.amazon.com/dp/B004KNPRBK/

Verify that the short cycling is caused by the Nest. Remove the red and yellow wires from the Nest and twist them together. Then monitor how the system is running. If it runs non-stop then install an isolation relay. If it doesn't then you need maintenance.