Soldered all components onto the Oshpark board (BTW, what a quality!)
Looks like I missed a pullup resistor for enabling Atmega328 reset pin being triggered by Serial DTR. Will add it to Oshpark files.
If you are seeing that your DIYArduino/Atmega is unresponsive via Serial although absolutely should work - check this pullup for PIn 1 on Atmega328, all pre-build Arduino boards should have it.
Everything else is looking good. I'm currently testing the board.