Voltage divider high resistace
You won’t find an Op Amp that meets your spec, so don’t try. Just enable by switching the low side with a PN2222A or an Nch . High side PNP driven by an NPN or a Pch FET with similar driver to a 1M divider when active. Calibrate it with a 10M load R or whatever is offered.  Add an RF cap to suppress noise.
This can be done with &lt;20 nA searching for the right low leakage Transistors and kept at room temp or lower in the off state.