I think you are missing the point that the most imprecise part in a machine is not the drilling but the welding. I am a maniac and it took a lot of time, so 1200x 600 machine i weld under 1mm in all directions. But it takes time, a lot of time to align and wait for heat... My big machine i did alone by myself and it was even more difficult. All is ~1mm plus one mistake at one end where ~2.5mm. How did i know- judging from channel depth when pouring the epoxy. Epoxy could be only 5mm deep and 3mm min, so machine must be precisely welded.

I doubt where i live / Spain/ a local welder could do that. Have been working in construction so i had to mount fence, doors, etc.

So as a conclusion, if sb welds for you, you have to specify one priority, not seek perfection. If sb welded for me, the priority will be for the top side / under epoxy/ to be in one plane as much as possible. Gantry though i recommend to be as perfect as possible. Z must be perfect /if welded, that means hard stops or fixture/

Anyway, all this said , then what it means is weld the plates, but drill them when on machine and rails aligned. Otherwise all must be machined.