I followed your steps and got DuckStation running fine in RetroArch after double-checking the BIOS location and the exact retroarch.exe path (that was my mistake at first). Doing a test load directly in RetroArch before setting it up in Attract Mode was key.
Also appreciate the note about ZIP files — unpacking them fixed a black screen issue I was having. For anyone stuck, definitely recheck spelling, semicolons between extensions, and make sure the DuckStation core is actually selected.
2v2 io (https://ozgames.io/2v2io)