If you want to disable the agc completely where it would be "Off" and the receiver at full gain, you can try cutting the trace from R1815 (22k) from Q1805. This resistor is in the base lead to transistor Q18, which is the AGC amplifier. With this resistor opened, no voltage from the AGC rectifier diodes can reach Q18 so it never attempts to lower the gain of the receiver.
My R7000 has nearly identical agc circuit and I did this modification and it works very well. I have not tried it on my IC-718.
If you want a "slower" agc for ssb, it will not be solved by simply adding more capacitance for C 1803. The high value of R1808 will keep it from charging or discharging rapidly. That's why you see so little difference between agc action on CW and SSB. The simplest solution that would require very few parts changes would be to turn the AGC into a "hang" type with a FET.