Motor Spool Time from min to max - how to slow it down?

AC 4.0.5
Which command is recommended to slow the motor spool up/down time when going from minimum throttle to full throttle?
Is it MOT_SPOOL_TIME or MOT_SLEW_UP_TIME or something else yet?

My copter uses relatively large gasoline engine with a 23" heavy prop. It hovers at 0.8 of full throttle. Gasoline engines, when loaded, do not respond to throttle stick as fast as electric motors. Throttle changes in all my auto-modes must be slowed down to about 2.0 sec from minimum to full throttle. If throttle changes too fast it even stalls the engine (it’s the nature of a carburetor).
I’ve read back from '2017 that a MOT_SPOOL_TIME_H was in the works but I’m not finding it.
I also see that the range for Mot_Slew_Up_Time is only 0-0.5s which would not be enough if this is the command to use.

Any advise?