Hey @pattern!
Looks like a very well planned out build!
Regarding the pro expander - I find when I’m building things this big, the power wiring can be a really big drag. It’s nice to be able to troubleshoot quickly and easily swap in good strips for ones that go out. The pro expander lets you do that. It can handle 15A per board, which is enough to directly drive ~1600 WS2815s directly off its pluggable headers (and more if you limit the brightness setting or never run all pixels at max intensity). It has a built-in 5V buck converter for powering the Pixelblaze, replaceable fuses, and several helpful troubleshooting LEDs.
Looking over your proposed wiring diagram (nice, by the way!) one thing jumps out to me, and that is possibility for a fairly low frame rate. That could be OK for a slow fading ambience project. The current-generation Pixelblaze v3 can generate about 48,000 pixels per seconds, depending on the pattern complexity. That works out to a little under 5 frames per second. On the forum we covered some more considerations for a similar build situation in this thread.
Your best options to boost frame rate are either to wire for symmetry (for example, if the left and right sides are always symmetric, you can split the data line and have half the number of pixels being computed), or use multiple Pixelblazes that are sync’d via Firestorm. Adding Pixelblazes should scale roughly linearly (4 Pixelblazes ~= 4X frame rate), but can be a little extra work to figure out if you need to rewrite patterns and coordinate your pixel maps to work well across the entire project.
Last, if you haven’t purchased your LEDs yet, consider the GS8208. We have some other threads here about them. On the LEDs Are Awesome FB group (and in my own experience), the WS2815s have a bit of a reputation for frequent failing, depending on the batch they were manufactured in. The GS8208s also have noticeably better brightness response curves.
Best of luck!