I am on at least my third darkroom configuration in the same space. I don't really consider my earlier designs to be mistakes. The earlier designs worked great for the work I was doing at the time. I made changes as my photography evolved.

I recommend that you make your design flexible. I don't use built in cabinets. I have movable tables, shelves and storage units. Even my sinks could be moved with some minor re-plumbing. I recently bought a used 8x10 enlarger that is much larger than my old enlarger. To accommodate it, I only had to lower a shelf on the work unit and remove the top shelf on the unit.

You might be able to build the perfect darkroom for your work today, but what will you be doing five or ten years from now?

I remember reading about a guy who build a table for himself. He ended up making three. He said, "I built the first table for an enemy, the second for a friend and the third for my self."