Is MIMEDefang an alternative to amavisd-new?
Yes. Both are written in Perl and both can use Apache SpamAssassin, rspamd and antivirus scanners, but MIMEDefang is a native milter. It runs your policy during the SMTP transaction, so you can reject mail before accepting it, with no backscatter. It also lets you rewrite any MIME part: drop or replace attachments, add disclaimers, or change headers.
amavisd-new usually runs as an after-queue content_filter. To migrate, replace the content_filter line with an smtpd_milters entry and port your policies into the MIMEDefang filter.
See MIMEDefang vs rspamd and amavisd-new for a full comparison and a migration checklist.