Welcome to cnc…everyone has a different opinion and as you can see, different numbers.
You have a ton of variables to deal with. You should just build something cheap and test it. When I used the Outrunners I had several sizes on hand.
The dewalt we use is 30k rpm.
You don’t have to use 12V. You should be more worried about peak torque than speed. Power is always the concern, don’t forget your gear ratio in the equations and losses. My calculations were off by an embarrassing factor with my senior project using an outrunner.The math is simple but the numbers given can not be trusted, at all. You can adjust to suit your rpm.
On top of all that your need to make sure you have extremely minimal slop/runnout in your entire system. It will be running continuously for many hours at high loads. Don’t forget to cover it to protect it from dust and dirt from cutting wood metal and plastic.
You have a long road ahead of you. It is entirely possible and will be simple when you find the right combination of parts, but I feel you will have a lot of real world experimenting to do.