Interesting. It seems like when you start with a huge CNC like this, you can go crazy looking for flat. My recommendation is to just bassically ignore it for a while, and get some through cuts cut. If you’re within ~5mm across the whole sheet of plywood, that’s usable. Get some CAM and setup practice, and then you can start optimizing the flatness. You’ll also learn a lot about where it matters and where it doesn’t and how to optimize your workflow to reduce it’s impact.
For example, when I did the carving on my MP3DP, I was using a v-bit to do precise carving for the logos, which requires very good Z. I used the same origin for all the cuts, but I split the carving into areas that were manageable (about 12" across). When I did my setup, I added an extra step of jogging to where the design would be cut, and finding a good Z value there, right in the middle. The error in Z was only the error in that area, which was very accurate for a 12" V carving. I proceded to “learn” a few other things :), but the flatness worked out fine. When I went to cut out the parts, I just added 2mm or so to the final cut depth, and any unflatness was just cutting into the spoil board. NBD.
All that is to say, you’ve got a great looking build, and as long as your not using a clearance plane of 10mm and cutting 10mm into the spoil board, don’t worry about the flat so much.