It’s pretty difficult to recommend. Just adding one new piece, I have someone hold it up for me and try different spots, as walll size/proportion and its placement height wise all come into play. Suggest someone help you and in regards to mixing, it’s your office so whatever you love, use.

The wall over the credenza could take something larger (taller), but since you already have the piece, go ahead and hang it. Just don't hang it too high. Even though your ceilings are high it should still be eye level and not look like it's out in space by itself. Your home office should be comfortable and I like to hang pictures I really like to have something attractive to look at for a visual break. I bet the top of the credenza is messy right now because you're in the middle of arranging everything. Once you're more settled you can work on having more decorative under the piece of art that would complement it. But in an office, function comes first.