postfix centered backup MX configuration ideas
No mention of redundant DNS? Backup MX is fine but with resolver failure even backup MX can be useless.
A more interesting idea (rough draft imagination) would be a symlink-analog from backup (third party) MX providers to access rules on primary (or local MX) such that rules need not be manually replicated.
While some find it rude I find TMDA efficacious (when properly configured)