Nice! Notes below.

How command line works is explained in Scripting API reference. There are more functions you can use than those listed in --help (it lists only the most important). Some examples:

copyq iconColor red
copyq iconTag 123
copyq exportTab default_tab.cpq
copyq screenshot > screenshot.png

If there is fixed number of function arguments you can chain multiple functions:

copyq iconColor red sleep 1000 iconColor ''

You can usually list available media types (MIME) using ? argument:

copyq clipboard '?'
copyq read '?' 0

Probably the most important types can be listed with:

copyq clipboardFormatsToSave

The PROGRAM argument in action is any command:

copyq action 'copyq popup TEST'
copyq action "ls $HOME/Downloads"

You can list the configuration keys with copyq config but not the possible values.

Sessions are used to start separate application instances. The default session is empty string, i.e. copyq show is same as copyq -s '' show.

This is more of an implementation detail, but to get list of running sessions, you could do:

ss -l -x | sed -n '/\/\.copyq[^_]*_s /{s/.*\/\.copyq-\?\([^_]*\)_s .*/\1/;p}'

# ... or "find" instead of "ss" (but it's less reliable since the config dir can be anywhere)
find ~/.config/copyq* -type s

This is how you can use the tab argument:

# read the first three items in "work" tab, separated by comma
copyq tab work separator , read 0 1 2

Completion script could go to shared/ (CMake files would need to be updated to install the file).

BTW, most of the functions need a CopyQ server to be running, except help, info, version and logs.