What command compares two directories recursively and validates that they are exact mirror images (if possible)?

The answer to this is rsync. Not just to do the copy as @nohillside suggests but it can be setup to do just a verification.

Normally I would say you need to get a newer version of rsync than macOS provides as the macOS one does not keep extended attributes but as cloud synchronizers don't either then that will not matter.

You can run rsync to do a dry-run, that is not to copy any files but just report the differences. The command is

rsync --dry-run -av pcloud-dir/ gdrive-dir/

A copy can be done by removing --dry-run This has another advantage over Finder in that if the copy is interrupted then you can just repeat the same command and it will not recopy the files already done.

Agree with @mmmmmm ; rsync is the better choice over cp

An alternative to rsync to determine

if there's a command in macOS terminal that will confirm that I have mirror copies in Google Drive of the pCloud repository

one can use the diff command. diff can provide a comparison of two directories and will return what exists only in one folder or another, or nothing at all if they are the same.

diff -r /path/to/dir1 /path/to/dir2

And, if @wide_eyed_pupil you are intent on sticking with cp, you can add some arguments to not copy files already in the destination and to preserve attributes and permissions.
For example:

cp -nRav

have a look at man cp to see what all the arguments do.

Also, of note from the man page..

COMPATIBILITY Historic versions of the cp utility had a -r option. This implementation supports that option; however, its use is strongly discouraged, as it does not correctly copy special files, symbolic links, or fifo's.

(use -R instead)