i don't want to discourage you but as i have some experience doing similar assemblies from laser cut parts:

-going from square to round rails is a very bad idea at that point. Why waste effort and money then to make all that rigid structure. If you go with round rails then make a simpler square profile structure, no need to make that laser cut monster

- BST automation at aliexpress , Fred is a very good seller, he could do all that stuff, machining screws, send 3m well packed ball screws, lower value, etc. tell him Boyan sends you, he gives me best prices compared to ebay and aliexpress, furthermore you could order all from him, including VFD ans spindle, he has more than he lists, just email him, no problem with errors or warranties

-the way you fix your motor and ball screw, that moves the Z left right, take care to be able to adjust that

-as i made some similar stuff i am telling you you will need at least 12 heavy duty clamps to be able to straighten all and solder. plus a big plastic hammer at least, i would say you will need a press. see what i did here for the Z #155 and imagine at your scale of things how difficult that will be in reality if plate is not straight after cutting