[DoridianParticipant]

According to the schematics, SPI1 is wired to MOSI and SCK of the Arduino with the two remaining pins not connected.

[DoridianParticipant]

I am currently running an NCM109, which, due to multiplexing, only allows each segment to be on 1/3rd of the time.
But, since I also have a “fade” effect to transition between numbers, even lower duty cycles should be possible.
To be precise on timing, without fade effect, I turn on a Nixie for 2ms, then off for 4ms using an interrupt timer (there is blanking intervals in between of 0.2ms each, but those aren’t necessary for your use-case at all).
Due to multiplexing, of course, I also do this by Anode voltage (as the Cathodes are all tied together).
If the DHV pin is actually a functional element on your Nixie, you can try working with it (but according to information I found online, you should not turn on a Nixie for less than ~1ms, so if you PWM on that, make sure it fits that), however I’d still recommend using the HV switch (the one you control with SPI) and not PWMing the supply.
However, I found out, on my NCM109 the DHV pin is just a relic from other firmwares that doesn’t actually turn on or off anything.

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