Hi Dingo- You are now talking about making a mill. That's a really big step up in rigidity over timber and plastic. You say "metal" is steel in those thought bubbles or only aluminium?

aluminium is maybe 5x stiffer then timber then steel is 3x stiffer again. So a steel machine has to be 15x stiffer than a timber machine (V roughly speaking). Then it needs more power to cut much much more power. etc etc

1) The 2mm section although it has a V large inertia is too thin to prevent the section changing shape under metal cutting loads (mainly torsion even for timber I expect). Typical steel gantry sections are 250x250x12 (because of your Z will need to be bigger). There is no gain in having the smaller section attached to the bigger section, you may as well put all the metal into one efficient section. I realise you need room for a drive screw you will need to figure that out but a solid is usually used not a hollow for this purpose. A 3m gantry is going to be a very big and heavy chunk of steel, aluminium will be better to save weight but it will need to be bigger as well either billetted and bolted together or welded sections
2) Your huge Z will be the crux of the design it will need to be steel 250x350x12 or bigger to be stiff enough. Have a look at blue elephant site and other builders to get an idea of their sizes for this sort of thing.
3) If you look up the Maximus thread it details designing an aluminium capable router from the start
4) I'd say your current design is under dimensioned badly for aluminium mainly due to the Z axis height
5) You have your long rails on the sides. Unless you have these surfaces milled its very difficult to get them parallel
6) If you are going to weld these long structures they will warp so you will need access to a heat treater for stress relief.

All of these things are in the many threads of this forum in great detail...

Whether you cut slow or fast does not matter if the machine is not rigid enough it won't cut. Mills weigh several tonnes in this size machine for good reason to get the rigidity...

Keep at it.... Peter