how to set up sharepoint search with metadata and categories

I have a sharepoint site for an accounting firm that needs to be able to search for documents with certain criteria:

Client Name, and Date.

First: no metadata is currently configured. Will someone need to go into each document library across the whole site edit each document to include metadata?

I know there is managed metadata where end users can define it while uploading a document, but i would like to avoid this as it is a burden for them. It also would not apply the metadata to all documents that already exist unless each one is edited then saved again...and there are hundreds.

What is the best way to configure this search for metadata for documents already on sharepoint, and from behind the scenes (no end user configuration?)

I have set up terms and term sets, but they want the option to search specifically for the categories listed above, not just by a term.

For example: Search for "Client Name, date" and results pull all documents associated with that client name and the date it was uploaded.

OR- is there a way to add filters to the search results like the basic Author and Result Type? One for client name and one for date?

Thanks!!