Is there any way to get the Kind column to just say RTF or something similar instead of always shortening to “Rich…ument”? Can I set my own abbreviations somewhere?
No you can’t change the reported Kind nor can you set your own abbreviation (which sounds like a recipe for confusion, if it was allowed! )
macOS does change the displayed Kind when collapsing or expanding the column, so there is a precedent. I will consider this a feature request - with no promises, of course.