I'd move the nest boxes to the outside. Also large, louvered gable vents on each end.
Definitely agreed - on both points! If you mount your boxes on the outside, you not only open up the inside space, but give yourself the luxury of adding an access door, so you don't even have to go inside to collect eggs. You probably don't need as many as you have now, either. Hens tend to share a favorite box or two. With 23, I'd guess four (or six, if you want to double stack them and keep the extra boxes) nest boxes would be plenty. Three of my ladies will actually wait in line for the "best" nesting spot, leaving the others empty. Too funny!
If you want to expand your space without adding a lot of footprint, try going off to one side (or end) with a small, people-height run addition - like a big, hardware cloth closet. That allows for a "people door" (and access to the lower run) and a nice, tall perch set-up (think cat-tree, but chicken-style!)
As for predator proofing, you don't need to dig up the whole outer edge for an apron. I dug down about 8-10 inches all the way around the base of my playhouse-coop conversion. One side at a time, as I could afford it, I filled each trench with dry concrete mix and hosed-and-hoed it until it was properly mixed. I liberally salted the mix with yard rocks to stretch the concrete, gave it a week to cure before dumping some gravel on top of it. So far, it's held off my 50 pound "digger dog" and the neighbors cat. Next Spring, I'd like to plant marigolds in front of it all, so my biddies can have a snack to graze on through the wire.
Oh - and I have one run wall that is still chicken wire. The next project is to tack hardware cloth over the inside of the support posts and fill the 3 inch space between the wire walls with fall leaves to form an instant, refillable windbreak/shade/composting treat. We'll see how it works!
And just so you know, officially, EVERY chicken coop is a "work-in-progress." With all of the great ideas people here on BYC are always coming up with, no coop or run is ever "finished!"