I'm looking for a term for describing this writeup of mine which is too long and detailed to constitute a paper, but seems to short to merit being called a monograph. Is there such a term?
A 50-page unpublished manuscript is just that: a manuscript. As far as I know, most disciplines don't have a publication format corresponding to this length, although even 100-page articles are not unheard of in some law journals. The exception are generic formats called reports or working papers.
Reports, as mentioned, are the most generic format, and they would accommodate a 50 page manuscript. (Technical) reports are distributed by their author's institution rather than published by a publisher. For example, they may be uploaded to some institutional repository, distributed to a number of libraries, or made available on request. Their content is often less polished, and more tentative or ephemeral than a journal article or monograph. They may go through some vetting, but usually no peer review.
Working papers are similar in all these respects, but they are part of a series that is supposed to be somewhat permanent. Some working paper series prescribe a page limit, some have more elaborate vetting, 'in-house peer review', as it were. I'm not sure, but my impression is that working papers are more of a thing outside STEM than within.
Both of these formats are considered 'grey' literature (although some working paper series might even have an ISSN), and it is understood that they represent work in progress that may become part of a journal article (or several) or of a monograph.