With an overarching plan established to set the direction of the design, it's time to actually produce a PCB layout and determine all the little ancillary components to support the main processors. 

So, following a hectic bank holiday weekend stuck in front of KiCAD and countless datasheets, I eventually managed to cram everything onto this board:

To give a quick overview of the specs:

• ESP32-S3 application processor with 4 and 2 MB of on-package flash and PSRAM, respectively
• RP2040 video adapter, connected to the ESP32 via QSPI at up to 320Mb/s, and a second full duplex SPI side-channel at up 80Mb/s (in theory)
• PAM8908 stereo headphone amplifier, chosen for being able to operate on a single supply rail without needing a bulky output capacitor
• Dual USB 2.0 hosts over USB-C (connected to the ESP32 and RP2040, respectively), using AP22653's as power switches with ~750mA current limit
• Full size SD card slot over 1-bit SDMMC interface on ESP32
• TLV62585 synchronous buck converter to power it all.

The board itself has a number of "interesting" design choices baked in to keep profile and cost to a minimum:

• Card edge connector on top-left adapted from peterburk's picoDVIEdge should fit into a HDMI connector without using a chunky socket
• Direct soldered 0.6mm thick USB-C 2.0 socket boards using 8-pin, 2.0mm pitch through hole interfaces

• "Novel" SD card and 3.5mm sockets using flexible PCB prongs (seen at the bottom of the board) which will be sawn off and soldered directly to matching headers on the card.
  • For the 3.5mm socket, a cutout in the board slightly smaller than the 3.5mm jack is used to keep it in place. The prongs are laid perpendicular to the jack, and press down on it to make direct contact with the rings while providing some retaining force
  • The SD card prongs are aligned with the contacts on the card, and each uses 0.8mm tall capacitors as pads to (hopefully) interface with the recessed contacts of the SD card. Cutouts for the capacitors allow the prongs to remain flat on the board when no card is inserted.
• 2-layer PCB coil for the buck converter - adds no thickness to the design, is free, and looks cool! (Though an SMD chip inductor would probably be far more practical...)
• Entire board uses only 2-copper layers, to take advantage of discount pricing offered by several PCB fabricators
  • Where traces have needed to be routed through the ground layer, I've tried to avoid routing underneath high speed traces (particularly the 40MHz QSPI and 80MHz SPI interconnects). These are then fenced with vias to a top layer ground pour, to try and bridge the ground plane over them. Hopefully this will ensure ground return paths are fairly uninterrupted.
• "Analogue" audio produced by the ESP32's sigma-delta modulator, passed through a first order RC low pass filter. This will probably sound "not great", but with only 2 passive components per channel it's also very cheap and compact.
• CC detection and power control on USB-C downstream facing ports to be handled in software on the RP2040. This allows common power switch ICs to be used, which were cheaper than any USB-C DFP controllers I could find. A mosfet on each CC line protects the RP2040 GPIOs from current injection when unpowered and another host is connected, or if connected to a host that pulls the CC line to +5V.

Putting all this together results in a board which should have no components taller than the 0.9mm-ish tall QFN packages, resulting in a total thickness of only 1.7mm!

With all the questionable features implemented, it's just a case of sprucing up the silkscreen and sending it off to be manufactured (no points for guessing where though!)

As it turns out, PCB art is hard, and not yet in my acquired skill set, so I instead resorted to using this cool font "AmazDooM" uploaded by Amazingmax. Thanks!