I use ChronoSync
https://www.econtechnologies.com/chronosync/overview.html on a Mac to backup 'everything'. Except I exclude caches and hidden files, and preview files I know apps such as Lightroom will rebuild if lost. This is to save time copying and space on the backup drives.
So for Photo Mechanic Plus I guess I need to backup all the files in each catalogue folder, catalog.pmdb etc. But proxies? I don't really need to back those up, do I? As far as I can see they're JPEG previews and if one (or all) are missing PM+ will just rebuild them?