You certainly can design or build it yourself, but you don't have to.

There's an absolute fuckton of new, offline 3D printers for sale in the world. If disconnected operation is the goal, then finding a disconnected printer to buy is a simple affair. Lots of them just take regular gcode (as produced by Orca Slicer or similar) from an SD card or over USB, and don't have built-in networking as a feature at all.

If having it run open-source firmware is also a goal, then that's fine too: Research it first to ensure that its stock controller board can be flashed with custom-built Marlin (for complete airgap) and/or Klipper (which tends to imply LAN-only connectivity).

Being able to run open firmware is more common than it is unusual.

It's a pretty basic research task to drill down and then pick one with the price/performance/community/support characteristics that you want. After that, just assemble the machine when it shows up, and then build Marlin or Klipper for it.

And people in the 3D printing community will help with all aspects of this process, regardless of how you set your own goals. :)