Okay makes sense. For the users who do want to go big. I thought about going 4×8 once upon a time… Could you drill a hole in the bottom center of your conduit (midway between your outer rails. Then tap it and run a piece of all-thread into it to make you a support that will still allow the top platform to roll over it? Increases your stiffness by a factor of 4 =). Kinda like the legs on the bottom of washers, and fridges, that thread in and out to adjust height.
Edit: And if that isn’t going to work or your still set on one size stronger pipe for version 2. Then you should talk to https://www.youtube.com/user/arduinoversusevil/featured . He is an engineer that travels the world working on things (oil rigs and such). He would know what is commonly available in most places, and what (if anything) would be perfect for your machines. You should reach out to him. He and his users would love your MPCNC.