A cnc router for cutting wood with 1200x600 (or 1000) cutting area is entirely possible with aluminium extrusion but would need to be the heavy gauge type and around 80x160 in section.

My machine has a gantry of about 700mm width, using heavy gauge 80x80 section and I would go bigger next time.

Recent designs on this forum have also used an 'L' shape arrangement from 2 rectangular sections to get stiffness in 2 axes, which is certainly worth considering.

If going for the raised gantry style then 20mm side plates are a minimum, but I would recommend going all out for the raised X axis style with no gantry sides. It's not much more effort to make and will give better performance.

You mention 20mm round bars not flexing much but I'm afraid you would be surprised how much they do flex. My very first machine had 20mm chromed round bars and matching linear bearings and was one of the reasons that some of the holes were a bit oval. Instead I think you would be OK with supported round rail for wood cutting, but would look for profile linear rail on at least the Z axis (short therefore cheaper) as it is vastly superior.

You also mention rack and pinion but I would stick with ballscrews all round personally. In terms of how the end supports work - one end (usually the driven end) sits in a housing containing a pair of angular contact bearings. When the securing nut is tightened this supports the ballscrew, allows it to rotate, but prevents axial movement.
The other end is just a simple support bearing and just holds the ballscrew in place whilst it rotates.
If this is a bit daunting then rest assured that you can buy a ballscrew 'set' with the ballscrew, fixed bearing, floating bearing, retaining nut (special) and drive coupling all included. You can even request particular lengths machined to your requirements for very little extra - have a look on ebay or aliexpress for examples.

You can cut aluminium profile on a mitre saw if you fit a TCT aluminium cutting blade. Take your time and it will go through like butter.