I always used L-brackets for my marquee retainers. While I agree, the retainers need to not cover the artwork, they can be the same thickness of the wood. I generally go with an L bracket that is 3/4 on one end, and a quarter or 3/8 thick on the other end. I try to make my artwork dark so it gives the illusion of a larger display area.
The only thing I see that you may want to change on your next build is to increase the viewing angle. Either by not placing the monitor so far back, or by giving it more of an angle (such as a wee-cade). My first bartop, I had a large marquee overhang and it caused me some issues since I had to view the bartop straight away.
Future cabinets, I started mounting my speakers on the bottom, instead of in the marquee area, so I could mount the monitor closer to the edge and reduce the marquee overhang.
Great looking build.