Pico 2 W wired straight to the cut DMG-04, no level shifter. This is the wiring that passed first light (1000 bytes, 0 errors). Hover a wire in the list to highlight it. Hole names are column letter + row number, e.g. a3.
USB end at the top. Left pins in column c, right pins in column h, rows 3 to 22. That leaves columns a, b and i, j free for jumpers.
Each wire goes into a free hole on the left (a or b) of its Pico pin's row.
| green, plug 5, clock | a3, pin 1 GP0 |
| red, plug 2, data from GB | b4, pin 2 GP1 |
| blue, plug 6, GND | a5, pin 3 GND |
| orange, plug 3, data to GB | b6, pin 4 GP2 |
Shield: twisted and taped, not connected. Nothing else is wired: no shifter, no power rails.
The Pico 2's RP2350 pins take up to 5.5 V while the Pico is powered (datasheet, FT pins), and the DMG reads the Pico's 3.3 V fine. This only holds for the Pico 2, not the original Pico.
Pico USB in first, then the Game Boy on. Game Boy off first, then USB out. 5 V into an unpowered Pico is not allowed.