Total noob about to embark on a build.

[Jonathan]

You don't want tension in a CNC machine because at some point it will release then things change shape.!!

Misleading, you don't want tension in a CNC Router. On a CNC mill or lathe it's quite the opposite - tensioning the ballscrews is necessary if you want the best possible accuracy, however you can't do this on a CNC Router as in general the frames are nowhere near strong enough. Applying tension to the screw would just flex the frame.

+1 to everything else you said.

I'd add in the cross pieces at the front and rear of the frame. I know it's nice to have one of those missing for better access, but it will severely compromise the rigidity of the frame.

You've stated 50mm aluminium extrusion - if you replace that with 50*50*3mm steel box section and weld it the frame will be strong and cheaper. Don't be put off by welding if you've not done it before. Welding my frame was the first welding I had done and it turned out fine...