I'm trying to make an estimate for self-discharge rate of a typical small capacitor

You should read the extended data sheet and look at the insulation resistance on page 14: -

So, I calculate the insulation resistance to be 10 GΩ using their guidance. This will determine the typical CR decay time (1000 seconds for a 100 nF capacitor).

What you linked to is what Kemet call their spec sheet.

after C1 is completely charged, we disconnect from the charging circuit and measure the V_output.

Most typical multimeters will have a 10 MΩ voltage measuring input resistance and, this would be a thousand times lower than the insulation resistance = ineffective test.

I'm trying to make an estimate for self-discharge rate of a typical small capacitor

You should read the extended data sheet and look at the insulation resistance on page 14: -

So, I calculate the insulation resistance to be 10 GΩ using their guidance. This will determine the typical CR decay time (1000 seconds for a 100 nF capacitor).

What you linked to is what Kemet call their spec sheet.

after C1 is completely charged, we disconnect from the charging circuit and measure the V_output.

Most typical multimeters will have a 10 MΩ voltage measuring input resistance and, this would be a thousand times lower than the insulation resistance = ineffective test.

Estimating self-discharge rate of ceramic capacitors from datasheet values DF and ESR

Dissipation factor is for AC scenarios and at DC, this would imply an infinite resistance because reactance would be infinite (\$D.F. = \frac{R_S}{X_C}\$). Insulation resistance takes over at very low frequencies because it becomes dominant.

Am I chasing rainbows or is there a simple way to estimate this?

You need to get hold of the extended data sheet and look for the insulation resistance on page 14: -

So, I calculate the insulation resistance to be 10 GΩ using their guidance. This will determine the typical CR decay time (1000 seconds).

What you linked to is what Kemet call their spec sheet.

I'm trying to make an estimate for self-discharge rate of a typical small capacitor

You should read the extended data sheet and look at the insulation resistance on page 14: -

So, I calculate the insulation resistance to be 10 GΩ using their guidance. This will determine the typical CR decay time (1000 seconds for a 100 nF capacitor).

What you linked to is what Kemet call their spec sheet.

after C1 is completely charged, we disconnect from the charging circuit and measure the V_output.

Most typical multimeters will have a 10 MΩ voltage measuring input resistance and, this would be a thousand times lower than the insulation resistance = ineffective test.