On arrival
A document is classified as it lands, and what matters about it is pulled out and attached: who it is from, what it relates to, the dates in it, the amounts in it. Nobody types that in twice.
Filing follows the structure you already use. Existing folders, existing naming, existing conventions, learned from where things currently sit rather than replaced with something tidier that nobody follows.
Retention and disposal
Each class of document carries your retention rule. When something reaches the end of it, disposal is proposed with the rule quoted, and, like anything irreversible, it needs a person.
Finding things
By what is in the document, by date, or by what it relates to, the order, the supplier, the contract. Not only by whatever the file happened to be called.
Access and sharing
Documents keep the permissions of the system they came from. Sharing outside the organisation is gated by a floor, at every trust level.
Point it at one folder and see how a fortnight of arrivals would have been filed.