Don’t get me wrong, but how do you think you are going to succeed in something like that where pros have failed over and over again?
Why do you think that the large mills working with steel etc. are made from cast iron and weight several tons? The Altmill and Onefinity work with alu extrusions and are a bit stiffer than the Primo/LR4, the PrintNC is built from steel and even that one barely mills steel. That’s as stiff as it gets for self-made.
Side note: Stainless steel is a bad choice for a printer, because they are usually welded and not completely round. For a CNC 0.2mm different are okay(ish), for a 3D printer that’s too much. Though it annoyed me a lot with the Primo and LR3, so I opted for DOM tubing for the LR4. Much better in this regard.
Also there is going to be sagging in the middle of the XY, which also does not work for a printer. In the other thread where you asked is a user that built a large format printer that works, but it’s a whole different approach (MPCNC Made In China: New Build!).
I mean, I am wishing you all the best, but please be realistic in what you can achieve. 