Designing a senvitive frequency to voltage converter
Solution 1) You could just use a frequency counter with a very large count. For example, if you sample over 1 second, your frequency counter would accumulate 5 million counts. If freq was 100 Hz above that, your count would be 5,000,100.

That would be an easy solution and I think most MCUs can handle 5 MHz timer inputs, or you can use a divider IC as a prescaler.

Solution 2) if you cannot do long sampling periods you can mix it down to a lower frequency.

Ok, this solution requires some fancy programming but is very reliable and does not drift like the analog solution you're proposing.

Basically, you use an XOR to digitally "mix" your input signal with a reference clock that is close in frequency to your input signal. This results in an output frequency that is the difference between the two signals.

This resulting signal you can then frequency count using any decent MCU timer block.

Here's a good illustration of what happens when you XOR two square waves together with slightly different frequencies and filter the output:



(from here)

So here's a system diagram of a possible solution:







simulate this circuit &ndash; Schematic created using CircuitLab


Use two of your MCU's timer blocks to create two frequency counters.
Use the first frequency counter to measure your signal frequency to a rough degree. Lower resolution because the frequency is so high.
Set the clock generator (via I2C, SPI, whatever) to a very exact frequency just a bit below your measured signal frequency. This frequency is very accurate.
Measure the XOR mixer output frequency. This counter can measure  higher resolution because it is at lower frequency. You can even add additional counter bits using SW to extend your timer resolution because it's running slower. Thus you'll be able to detect very small changes in frequency because the small change is now a large change of the mixer output frequency. (This is basically same as radio signal demodulation.)
The exact signal frequency of the input signal can now be calculated as the clock synthesizer frequency plus the frequency measured by the second counter.
Track slow changes in input signal frequency using step 2 and 3 above.