Building on a Master 3X frame, why is the stack area so cramped?

On this frame, cutting the motor wires to length is pretty much the standard move. I fought the same bunching under the top plate on mine and eventually just desoldered all four motors, held each one in its final position, and cut the wires so they run almost directly to the pads with maybe a few millimeters of slack. The rear pair on the Master 3X design comes from the factory way too long, like they expected a bigger layout. Leave a tiny bit of extra length so you can get the arm off for maintenace, but not enough to fold over. I also twist each phase pair lightly, which keeps them tidy and lets the bundle sit flatter against the arm. For the front motors being stretched, double check you haven't got them swapped or crossed. On mine the front wires actually had plenty of reach once I routed them around the outside of the standoffs instead of straight over the middle. Going around the perimeter buys you a surprising amount of length.

Under the top plate itself, the trick is to plan the wire paths before you screw anything down. ESC to FC harness folded flat first, then RX and VTX wires tucked along the edges rather than across the center. If something still pinches, thin foam tape on the underside of the top plate stops the rubbing without eating much space.

Camera area, I feel you. My antenna also wanted to occupy the same airspace as the GoPro mount. What ended up working was a small right angle adapter on the VTX, then running the antenna back and slightly down along the rear standoff, zip tied to the top plate edge. It leans a little but the range has been fine and it's out of the prop wash and out of the camera's frame line. Some people just angle it back at 45 degrees and call it done. And no, you probaly didn't do anything dumb earlier. That frame is just cramped by design. Everyone I know who's built one said the same thing, that the top plate goes on last and with a bit of swearing.

Same experiance here, going around the outside of the standoffs was what saved my front motor wires too. I routed them under the arms instead of over the frame and suddenly had more than enugh reach, no re-soldering needed at all. For the antenna I did something a bit diffrent, I slipped a short piece of silicone tubing over the zip tie where it clamps the antenna to the rear standoff, stops it getting chewed up by vibration and I can slide it up or down to tweak the angle. Slight lean backwards but it's been solid through a bunch of packs.

Not sold on the twisting advice tbh. I twisted the phase pairs on my last build because someone swore by it, and all it did was make the bundle stiffer so it refused to sit flat under the plate. Ended up untwisting everything and just running each wire flat with a dab of hot glue at the arm root to stop them buzzing against the carbon. Zero issues since, and the ESC wires lay way lower. Maybe it works better on bigger arms but on something this cramped the twist just adds bulk where you have none to spare. Did yours actually sit flatter after twisting, or did you end up rerouting anyway?

One habit that's saved me a few rebuilds, leave the top plate screws finger tight untill the whole quad is powered up and bench tested. I built four of these clone frames for guys at my local field last season and every single one needed a wire shuffle after the first power on. If you tighten everything down before that, you just get to undo it all again. Also keep an eye on VTX heat once that plate is on, mine ran noticeably warmer with the top plate trapping everything, so I dropped it to 25mW for bench work and put it back up when flying.