Your wall with the picture should hold a large hutch. A hutch would hold dinning and office material.

Dinning hutch or buffet cabinet will go perfect against the long wall there:

https://www.houzz.com/photos/shady-view-kitchen-traditional-kitchen-minneapolis-phvw-vp~526726

https://www.houzz.com/photos/project-in-tiburon-traditional-dining-room-san-francisco-phvw-vp~298647

just the buffet base of the hutch is all you need. and just because its a buffet base doesnt mean it has to have platters and napkins in it...

see this antique tibetan buddha cabinet? who'd ever guess that the bottom is stuffed with yarn, knitting books, and candles and coasters.

If you keep the top shelving open, you could have both additional storage and keep the open feel you are looking for. Could paint white to match your existing kitchen cabinets too, to give a nice continuous feel. You could do a sideboard but I feel the way the space is laid out, it is just made for built ins, since it is framed nicely by short wall space on either side. By going wall to wall you will really maximize storage and if you mimic the kitchen cabinets it will look like it really belongs.

A buffet piece to replace the cart. Just look for one with lots of storage. Probably avoid glass front to reduce visual clutter. Center on that wall, then add a *few* decorative pieces to the top and it will give you the storage you need without feeling too heavy.