Each .mc file is compiled into a configuration file usable by sendmail. (The editable .mc files are nearly as unreadable to humans as the resulting .cf files, but sendmail cannot read .mc files directly.)
I don't know what FreeBSD has, but OpenBSD had a selection of distinct, separate .mc template files. You would pick
one to use, and then adapt to your needs, then make(1) a single .cf file from it for use with sendmail.
Sendmail is one of those applications which has both
an open-source community that supports the application and a commercial company that offers commercial products and
commercial support. As you seem to be locked into sendmail, please consider engaging one, or the other.
I have not used sendmail in many years, and hated its insane provisioning structure when I was a user. Unfortunately I cannot come to your rescue, as I don't use the software and have never used your OS, and what little I've told you about sendmail hasn't seemed to address your needs, or get you to move to something else. Anything else.