There should be an option to configure the default MX record(s) to use mail.domain.com, the server’s hostname, or none, and maybe a textarea to add additional mail servers and priorities.
See attached screenshot for my idea of how this could work
yes you can. Just add your default MX record in that field. It will apply to any newly created server using that template.
If you want different settings then create new templates with the other settings and name them logically.
That’s not true. Even if you add an MX record, it still adds 5 mail.domain.com. as well. The only way to get around this would be to check “Use only the records above” and add everything manually.