I’ll add a THT 5×20 mm fuse holder and M3 mounting holes, insert the fuse into the 18 V input path, resize the board below 80×80 mm, compact-place everything to the enclosure edges, then reroute and verify.
The fuse is now electrically in series, the compact Nano and flyback footprints are in place, and the board is resized to 79×79 mm with four 3.2 mm corner holes. I’m running the dense placement pass now.