After my design considerations the other day, I started designing a schematic to reflect them. I created it in KiCad, and it's currently made up of two sheets. One contains the microcontroller, power supplies, and RTC; the other has the Numitron digits and their respective controllers.

While I was creating my schematic, I had to change some things from my initial design. First, I realized that I wanted to be able to vary the brightness of the Numitrons. I did this by creating a separate voltage supply, and making it adjustable by way of a digital potentiometer. Secondly, I wanted battery backup for my RTC; the easiest way to do this was to add an external IC (the DS3231) that supports battery backup from a coin cell.

I'll be testing this circuit on a breadboard within the next couple of days. If you'd like to view the current schematic, refer to the GitHub repository that I created to store the project files.