Is this one of these projects that are never finished?
To a point.
A number of factors go in to this for me.
First, I don't usually know what I specifically want. I know what I want in general most times though. Trial and error narrows it down. Second, I have to find the art that supports or comes close to what I'm thinking about. That usually takes a while and influences the first point. Sometimes it's a very long search. Third, in relation to the first point I often have to trial and error
elements of the art I find in order to narrow down what I want.
Finally, things look different outside of Photoshop and sometimes there are new influences that come up later on.
I have a unified theme/purpose in mind for at least two Macs because my time is split between them for the content I create. The theme really only matters to me, but inspiration influences creation. Anytime I spend 8-15 hours in front of a monitor I like to be inspired by the backgrounds - not bored or frustrated by them.
Once I get there though the wallpaper ceases to change. I've got a few Macs where this is the case and my iPhones. Notably my 17" PowerBook G4 has had the same desktop background since around 2014-2015.