I find that the spool up options of "slow" can be very slow and "fast" can be too fast for some heli/ESC combinations. An option in between would be perfect.

Unless there is another way to slow down the early part of "fast" spool up to give the ESC more time to get through it's own soft start?

The issue could happen when ESC spoolup will interfere with Spirit Governor spoolup.
If you will turn off the ESC spoolup procedure then difference should be only decent between two nearby options.
Some ESCs are not working well with external governors in Heli modes. On the other hand some have problems with tail kick during spoolup in plane mode.
So it could be enough to try different modes recommended by ESC manufacturer suitable for external governors.
Unfortunately not all ESCs can be tuned for perfection in spoolup with external governor.
More information about your setup would be helpful if you wish to help. Adding slightly different spoolup rate will very likely not achieve what you wish in this case. Please check if you can observe big difference in Plane mode.

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Spirit System developer