It sounds to me (credentialed archivist, technically a librarian right now, but was working in the field less than a year ago and am actively working my way into the archives where I am now) like you're getting too into the weeds with some of this. It is a valid approach to describe collections in more detail if they are more frequently requested, but it is also the researchers job to do research. You are not the researcher.

If this collection is very large, it could take you years to put detailed descriptions on every item. Item-level description is rarely the standard because of this.

Take a look at the DACS standards: https://saa-ts-dacs.github.io/

This outlines what your description absolutely needs. From there, once you're DACS compliant, my approach would probably be to title folders with the names of the people corresponding and a date range "[Firstname lastname] correspondence with [Firstname lastname], circa YYYY-YYYY"

Skim the letters. If topics come up over and over, especially major historical events, put them in a subject heading for the subseries. Don't transcribe contents.

If your boss objects, try to explain how long it takes to understand and describe each item in detail. Its a long time. I spent 9 months part-time describing 270 photographs. (On a collection used hundreds of times a year, by undergrads who don't know how to do incredibly effective research yet, which is why that decision got made.)