For proof of concept NodeMCU was connected with a DS3231 RTC module on a breadboard. This was enough to start working the firmware. Later, a PCB was created. The PCB is basically a re-packaged NodeMCU design connected with DS3231 and NCH8200HV high voltage boost module board. Progression of the Nixie clock control board from protoboard to final PCB

Schematic

ESP8266-based control board schematic for single-digit Nixie clock

PCB

ESP8266-based control board PCB for single-digit Nixie clock