Something to ponder…Re: adding doors:
What you created is very similar to what we did in the existing wardrobes in our LY. We added shelves & totes. The doors were already in place. Using your picture as a reference, the hinges are on the right side, handle on the left, and the door opens against the wall. Our doors cannot fully open as they hit the window valances. It's hard to tell from your picture, but it looks like you might have the same situation with a door stopping at/hitting the black thing on the wall. The real problem with the door not fully opening, is, the totes can't be pulled all the way out….making the contents more difficult to access. You mentioned easy to get to storage…so, I thought I'd give a "heads-up" on what you might encounter if adding a door. BTW, our "solutions" was smaller totes that can be easily removed. They don't fit the space as perfectly, as yours do…but the space around them is still good storage.
In rereading this, I guess the hinge could go on the other side….or top or bottom…or a fabric roller shade…or. There's probably someone on the forum who has figured this out.