It seems to me that you could simply rectify and filter you 6VAC ( or something close) and be done with it. The rectified DC will be proportional to the input AC, less the rectifier drop which will be quite small compared to the rest of the signal
You will need a load resistor across the filter cap. Since you are handling thermal, the value can be quite large.

However, why are you interested in the input voltage?
If the temp drop because the input voltage is lower, your temp sensor should tell you to increase the firing angle.

Erik