It seems that we have different setups, I am using only the courier-imap with postfix and maildrop while you seems to be using the full fledge courier. I will stay away from any conjecture of how you should setup your system because you only provide information about the filtering scripts. However you might still get some useful ideas from the information that I have provided to others. See it below.

I would like to call your attention to the fact that you want to stop delivery
of spam email. Are you providing email service to yourself only or to paying customers? The reason that I ask this question is that, if I understand it correctly, stopping delivery of email, even if it is spam, can be unlawful. To avoid any problem with the law I deliver all email, but the one that is detected to be spam, or has a small degree of being spam, is delivered to specifically labeled folders.