Is there a reading index or grade level index available to check documents?
Had a quick look around and found an old tool for OpenOffice which seems to work with LibreOffice and includes various metrics including Flesch, Kincaid, FOG, SMOG, ARI, Weirdness, coherencence and Brain Overload.
See attached example readability analysis to see if it is what you are looking for.
Readability report extension is quite old but seemed to work okay on LibreOffice v220.127.116.11 (64-bit) Readability Report | Apache OpenOffice Extensions
note: The extension did not seem to like frames and did not seem to include some text within the frames, It may be suiltable for straight paragraph text. I have not done extensive testing myself.
Before relying too heavily on it I would definately do some testing. At the very least I might suggest say 15 tests. Say three people each try it out and use the extension with five documents each, then proceed to manually grade the document aftwards to compare the generated scores to the manually calculated score
If this answer helped you, please accept it by clicking the check mark to the left and, karma permitting, upvote it. That will help other people with the same question. In case you need clarification, edit your question (not an answer) or comment the relevant answer.