Hello William - Welcome to the hornets nest. Your in very early days in your research and capability. You need to study commercial mills of the size you propose. You are not talking about a router...I think you will find them to be much more substantial then you think. Personally and I have put a lot of vibration engineering analysis in these threads filling steel tubes with FG is only adding mass not much damping via material damping. I looked at your chart and you note CF as 400GPa stiffness. This is a high modulus fibre std modulus is 200GPa and HM fibres will be extremely expensive. Plus it will translate into a laminate of around 180GPa max (at say 1550kg/m3) by the time you build it at 55% fibre volume and have enough cross ply to provide torsional stability. With std CF you get around an 80GPa laminate with 210GPa fibre. It is quite difficult simulating the dynamic response (damping) of a multimaterial multipart structure unless you have specialist software. Doing a modal analysis is quite easy in most std FE programs.

stiffness and rigidity - go big very very big and thick. Thin sections vibrate
damping - exceptional vague area even for the experts. Aluminium and steel are very live, damping ratios in the order of 0.05% so need lots of mass to mitigate
manufacturability - all things are buildable, depends on your budget, available resources and the desire to do something
MMR - you need to specify the MMR you want then look at commercial machines to see what they do. Do not think they weigh 2 tonnes or 15 tonnes for no reason. KE=mV2 so the faster you go the forces are exponential and the more mass you need to oppose the inertial forces F=Ma and then you suddenly need tonnes of mass so the machine does not skip across the floor. Been there done that with robots...
Beam sections - will need to be closed sections to maximise torsional stability and local stiffness, your current approach will not have the torsional rigidity to get what you want. Current research indicates an outer shape of a square with an interposed circular section within to be the ultimate shape. The square outer for bending stiffness the circular centre for torsion of which there will be heaps required where your going
machine stiffness - initially you will need to decide on a static stiffness for the machine typical VMC's are in the order of 100micrometer per N stiffness plus 100um/N at the tool. very small hobby mills maybe 10um/N but their MMR is tiny. Your FE model will overestimate stiffness by about 50% as a built structure. Part by part will be Ok...
easy to build - if you have a commercial requirement for the machine then buy a mill, where you are going is not easy, be prepared for pain. You have about 2 years to go before you will be capable of completing this project. You think you can build a machine in a few months then a year goes by and another... unless you are employed full time to get this done (and you have reasonable mechanical and electrical engineering experience) then a year maybe...

You need to specify your cutting forces, rapid speeds and various mechanical electrical requirements (and then validate them via commercial benchmarks or the knowledge in the forum) before you do too much more FE stuff. Plus a build budget, ultimately the $$$ will count. Do lots of searches for mills in this forum you have lots of reading to do.

Oh people here can't help you much with gantry advice because you have not told us the size of machine you need. Gantry types depend on whether your making watch parts or airbus wings. So a spec is important please...

Unless you know what the target is you can't shot at it, so a written spec is very important. My day job is designing and building machinery so I do not say these things lightly... Good luck and have fun Peter

By the way it is very difficult to guess at what is optimum in a structure. If you are using fusion360 or similar they have form finding algorithms which will produce near optimal geometry in one hit, no need to guess.