Before you start designing improvements, try out the current design. There are so many things to learn about CNC having a “smaller” one to start with is probably a good idea.
I am always trying to come up with ways to improve this. Right now rigidity isn’t my main concern, its actually a single machine that works with multiple size rails. So instead of adding pipes, bearings, extra hardware for rigidity you can just get a larger diameter. It will also make this Australia friendly. They seem to be the only country that can’t get 23.5mm or 25mm pipes.