I am not sure where 0.2V drop came from, this MOSFET has very low resistance (0.05ohm) and with typical load less that 20mA drop will be less than 1mV.
Output voltage value is not critical and does not have to be exact 18V, it can be as low as 14V for load to operate correctly and it works.
Device has occasional use, few minutes per week max. So battery life is not critical at all.
From what I know with previous designs with a mechanical power switch that batteries die because of age first.
My goal is to have a reliable device and reduce number of tapes used because of SMD machine limitation.
Adding another PNP transistor is not an option, IRLML6401TRPBF is already used in polarity protection, but I may use IRLML6402TRPBF instead, with higher GS voltage.
Also I like the idea of adding another 80K to G-S,
Will it be better is I use R21 3K and Rgs 10K to create 4:1 divider?