I’ve tried uncommenting the relevant line (exec --no-startup-id picom -CGb) but nothing changes. Also tried using xcompmgr but that didn’t work either. The only way I’ve gotten it to work at all is to manually run picom & in the terminal, which of course stops working once that terminal is closed and is not how it should work.
I ran EOS i3 for a couple of months back in the spring (on the same PC) and didn’t have any issues with this, so something in the newer default setup must be causing the issue.