Your 3018 has a GRBL controller, which is mm native. Modern versions of GRBL should properly interpret G20/G21 commands, but it is probably best to use a GRBL mm post processor. You can design in inches, you CAM should be able to do the conversion for you. The depth of cut, as well as many other parameters (bit size, feed rate, spindle rpm, cutting inside/outside/on the line, ...) are all defined when you create toolpaths in your CAM software before exporting the toolpaths into g-code.
You need to learn and understand the entire process to go from a design to cutting on the CNC, it is not draw push a button and go.
Tubular latex pressure vessel configuration engineer