No need to restart spamassassin, what is needed is sudo systemctl restart spampd
You should be able to see the result in the email headers you send.
Some debug can be added to spampd by configuring /etc/default/spampd. Look for ADDOPTS and set it to ADDOPTS="--debug --log-rules-hit", then restart spampd again. I’m not sure if this ends up in mail.log, otherwise look at journalctl -u spampd