I think I may have finally gotten it. Currently in the process of testing. I'll post back if this works or not, because it'll be good to have this configuration available for the next struggling person. This is what I have come up with so far (some things may be commented out so I can easily recover from "disasters":
SSLCertificateFile /etc/ssl/mx_mydomain_com.crt
SSLCertificateKeyFile /etc/ssl/mx.mydomain.com.key
#SSLCertificateChainFile /etc/ssl/PositiveSSLCA2.crt
# smtpd_tls_cert_file = /etc/ssl/PositiveSSLCA2.crt
smtpd_tls_cert_file = /etc/ssl/mx_mydomain_com.crt
smtpd_tls_key_file = /etc/ssl/mx.mydomain.com.key
smtpd_tls_CAfile = /etc/ssl/AddTrustExternalCARoot.crt
smtpd_use_tls=yes
ssl_cert = </etc/ssl/mx_mydomain_com.crt
ssl_key = </etc/ssl/mx.mydomain.com.key
# ssl_ca = </etc/ssl/AddTrustExternalCARoot.crt
ssl_ca = </etc/ssl/PositiveSSLCA2.crt