Should I use MIMEDefang or rspamd?
You can use both. rspamd is a fast spam-scoring engine. MIMEDefang is a mail-policy framework that lets you write per-site rules in Perl over the envelope, headers and MIME tree.
MIMEDefang can call rspamd through rspamd_check and act on its verdict: reject, tag, quarantine or rewrite the message. If you only need spam scoring with a web UI and no code, rspamd on its own may be enough.
See MIMEDefang vs rspamd and amavisd-new for details.