12mm is actually .472”

This is what I used and all the mechs from pinball life work fine at this thickness

It’s all in how you program the machine and set your tools.
Most would set up the top surface of the wood as Z zero and program all the paths as negative numbers in Z.
If done this way you don’t need to worry about how thick the playfield was drawn in CAD when it comes to the inserts because they will be releative to the surface .
What’s important is when cutting full depth you program it the exact thickness of the playfield so you don’t cut into the waste board.

Some program z zero at the bottom of the playfield. All z values/paths would be positive. (or zero when cutting full depth)
To me this is wrong for a lot of reasons.
#1 being more likely to crash the machine. But to each their own.

I would program the inserts .245” depth.
Leaving .005” to sand.

I would skip fractional and go inches with decimal point.