For Purcell factor, if you use the result from the dipole source, or using dipolepower/sourcepower, when the mesh is finer, or the dipole is located at a lossy material, the analytical dipolepower is not accurate: https://optics.ansys.com/hc/en-us/articles/360034925293-dipolepower-Script-command
You will need to use box monitor to modify the actual dipole power. If it is in lossy material, the box monitor dimension needs to be carefully set: too large the dimension will lower the actual radiated power due to abosorption; too small the dimension may affacts the accuracy of power integegration.
As for the mode expansion monitor, there is a box "frequency points" where you can specify the number of frequency points to match those in DFT monitor. Sometimes the forum does not accept the image so I will try to paste it in next reply.