I wouldn't qualify because I don't live in the Netherlands and, anyway, I do these things more out of interest than for money because I find Nixie clocks fascinating. I've built three and have published one here, incidentally a 6 Tube ESP8266 based one Six Digit Nixie Clock . I've also got the tubes in stock for two more 6 digit clocks so I have something to do during the next lock down.
I guess, at least for the basic hardware, you are working based on a ready made design. Multiplexed designs require less hardware but are more complex to program and the perceived display brightness maybe less than that in a non multiplexed design.
Anyway, it is best to develop the project in such a way that there is some testable and visible result at each stage and there are a number of milestones.
On the hardware side is getting the high voltage power supply up and running and demonstrating that the nixies work. Simply connect one anonde, via the current limiting resistor to the power supply positive and systematically ground each nixie cathode to check it.
Purely in software, you can test getting a time from NTP using the ESP8266 and displaying the results on the serial monitor. This, incidentally, can be done using the newish inbuilt NTP functionality of the ESP8266 in an incredibly compact few lines of code and returns results in your time zone.
Then there is a bit of joining up to do starting by developing a function which will, given 6 (or maybe 4) arbitrary digits, will display these on the Nixies. This interfacing to the driver chips can be one of the more complex parts of the project. Finally, putting it all together.
If you decide anyway to progress further yourself, just open a thread, say under Project Guidance, and include your schematic and any code you've developed and you may get enough suggestions to help finish the project yourself.