What's wrong with the Xduoo X3?
It works with Rockbox[0], has a monochrome OLED screen, can take 2 MicroSD cards, and has two 3.5mm sockets: one for line out, and one for headphones.
The only thing I don't like about it is the headphone socket, which is prone to fail. It feels tight on insertion/removal, and the extra force can eventually cause it to lose connection with the mainboard.
They cost about $100 new, or half that used.