Populating a 6x34mm PCB with 0603 components on both sides isn't easy and the result isn't pretty. For the next batch of 5 PCBs I'll try some sort of reflow process.

Bidirectional RS485 communication works (i.e. the Forth command line via RS485).  I didn't populate and test the DS1621S chip - a little suspense can't do any harm ;-)

Cables from left to right: RS485 (B is white), power supply (+5V), simulated UART for primary console (PC5), SWIM (PD1). Using SWIM while PD5 is in "half-duplex UART Rx mode" works fine.