I use a single monitor and find myself with a lot of random Finder windows open. I regularly navigate to a specific folder, open the file and then the Finder window stays open in the background. This happens hundreds of times a day and I’m regularly opening / closing these windows, which then sit behind other opened apps.
Has anyone got a suggestion on how to better deal with all these windows?
FYI… I’ve tried using Spaces but this has always felt a bit clunky to me. Happy to be convinced otherwise though 
Hold down the Option key when you double-click the file -- boom, and the window is gone...
OK, so not a KM solution -- but it is an easy fix!
Another option would be to simply quit and reopen the Finder if too many windows are open. This would even be a KM solution 
I think if done well a shortcut (when pressed) could quit the Finder and (when released) start the Finder again.
Or very simple 
Or you could just Option-click the Close button on any Finder window to close them all... If you really wanted to KM that then send ⌘⌥W to the Finder or "Select menu in Finder... File... Close All".
There are reasons to relaunch Finder, but this really isn't one of them!
Ah, yes that's even better.
What are the good reasons to relaunch the finder?
Same as for force-quitting and relaunching any app (which is what you're doing) -- "misbehaviour". A common reason would be when you open a window and you get a spod[1] but no contents displayed and no ability to switch to other Finder windows. You might see that with a dodgy network server connection, for instance.
[1] "Spinning pizza of death", for those who don't know 
Add ⌘⌥O to the list.
