Hey Wayne, welcome to the party! There are a lot of great folks here. Probably one the the best forums on the internet, period.
I did not pursue this so I can't answer for it. I was looking to KM to move files for me as updating to Catalina broke Hazel which was my go to for moving files. Not long after this question, Hazel was fixed for Catalina and I just went back to it.
I had the thought that I'd simplify my setup by using KM for all my filing needs and get rid of Hazel but Hazel is so easy and good that it doesn't make sense to reinvent that wheel using KM.
There are some coding wizards here that could probably do it using code to extend what KM has internally but I'm a coding baby and not interested enough to invest learning beyond my immediate needs tho I have poked at it a bit cause learning is so damn great too!
Hazel is excellent for moving files and has (like KM) a smallish learning curve to get it up and running.
In a word, simplify. IF (big if perhaps) you keep both Desktop and Downloads folders empty and not use them as folders to store stuff but rather as temporary work spaces, then why have two places to look? I have Hazel move everything that lands in the Downloads folder to the Desktop so I can forget about the Downloads folder altogether. One less drawer to junk up AND it boxes me in to getting stuff filed.
As a job will fill the time allowed, files will fill the space given.
Of course everyone has their own thinking/organizing/working/flow so this will not work for everyone.
I understand what you're saying. I do similar, sending all downloads to a volume mounted using the memory card slot on my Macs. What I'm wondering is, why have Hazel move the files from one place (Downloads) to another (Desktop) when a desired location for downloads can be specified within apps? Clearly, what you're doing works, it just seems unnecessary to add in another app to achieve the end result.
Simple, I never thought about it. Once I had Hazel I set it up and didn't look back.
I know Safari can target downloads to the Desktop. Aren't their other apps that target the Downloads folder and aren't there other ways things get there? I never thought through it. I just realized one day that my data was scattered around, started consolidating and found the Desktop to be the only place I couldn't eliminate so worked to eliminate everything else.
Hazel also makes available eliminating Mail's duplicate file structure by dragging all my email to the Desktop (using KM, natch). There Hazel files a great deal of it into my primary filing system (Finder files). What Hazel doesn't file gets addressed by keeping the Desktop cleared and Finder tagging and Calendaring what needs attention.
I find the exercise of developing a taxonomy a great practice to develop my thinking.