I am pretty happy with the Spirit. I love the reliable rescue feature which has my back when I need it most. The Jeti integration makes it even more enjoyable for me.

I am now trying to fine tune. The governor needs a little fine tuning to keep the RPM a bit more consistent, and the tail kicks whenever I apply collective. I understand that this is a "trial and error" process.

When optimizing the rudder, when do I adjust the regular "Rudder Gain" vs the "Rudder Common Gain"?

As I try to figure this out, it seems to me that the Sensor >> "Rudder Gain" is just a pure P gain, the Advanced >> "Pirouette consistency" is a pure I gain, and the Advanced >> "Rudder dynamic" is the D gain. Assuming that I am correct and that all three PID values can be adjusted independently, what is the purpose of the "Rudder Common Gain" multiplier?

I've found that for larger helicopters or for super fast servos you end up maximising the rudder gain on the transmitter. Hence if you need to go higher you need to use the multiplier (rudder common gain) to give you more headroom to adjust gain in the TX.

You can find explanation for all parameters when you hover the cursor over the parameter and also in the manual.

Spirit is not using PID, the algorithm is much more sophisticated. It is better to forget everything that you know from VBar as it can lead to situations where you try to fix something but it will not work in that way.
But Pirouette Consistency is really similar to I gain, Rudder Delay to D gain. I can tell you what to set there according to your setup.

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Spirit System developer