I didn't see any discussion of how frequently data was being added to the archive store; that's an absolutely critical point, especially if there comes to be a gap: quarterly backups of archive, backups of the main store kept for, say, only 8 weeks... a problem just before the (not unlikely to be delayed; after all, it's "read only") quarterly backup and you completely lose at least 4 weeks of data from the first part of that quarter.

Further, I disagree about a quarterly full and daily differentials being a nightmare to manage; you're talking about less than a hundred backup files. If we can manage millions of rows in hundreds of tables, then surely a hundred files in a single directory path isn't that difficult if you take the time to set up management, validation, and restore test protocols in the first place.