I'm realizing what an utterly wild English language word "psychophysics" is for new learners. It's so many conflicting sounds and rules in one.
That's not to say that it's not very tricky, just that the difficulty kind of arises from one core reason.
Edit: OK, one s as /s/ after the c and another s as /z/ between vowels is also tricky and doesn't have to do with Greek.
The English plural suffix follows a standard pattern:
It’s voiced (/z/) after voiced sounds and voiceless (/s/) after voiceless sounds. That’s why it shows up as /s/ at the end of “physics” (/fɪzɪks/). /k/ is voiceless.
The -ics ending of physics comes from a plural.
No helpful pattern underlying the middle “s” unfortunately.