That was the goal I tried to achieve when I built my first machine.
The way I did it was to make a drop table, and having several different Z axis of various lenghts. a long one for 3D printing, a shorter one for milling, another one for plasma cutting, etc.
This worked fine, no major issue. I was getting relatively decent prints and also relatively decent milling. I was able to mill wood without any issue and also aluminum, although my results with aluminum weren’t that good. I understood later that it was due to my spindle not being powerful enough, so I suggest to get the most powerful one you can afford if you want to get better results, in my opinion spindle power matters way more than machine stiffness.
You can have a look at my old machine here in the following thread: My MPCNC made in China
In the end it was a bit of a hassle to switch between the different systems every time, so a few years later I rebuilt the machine entirely to make it 3D printer only and bought a 3040 CNC
I hope you’re a real person who really intend building something and not a useless AI bot intended for making us lose our time.
Good luck, and if you want to lift all doubt the best way would definitely be to share some pics of your build.