Reducing the brightness of Nixie tubes
Don't mess with the voltage, that will cause uneven lighting.

Generally for glow discharge tubes like nixies you can get much better control by PWM than by messing with voltage, so I would suggest programming the tiny to PWM the A1-A4 pins. You will want to make the frequency high enough to avoid quench effects (still a thing even in glow discharge regime) and audio frequencies (Actually it looks like 100-500Hz also works because the time constant is really long).

An explanation is given at Wendt's page. He used a 555 to generate the PWM, but the tiny probably can just generate it internally.

As a bonus, you can change the brightnesses on a per tube basis.