Ideal hardware for a high precision 5/6 axis CNC machine

Ideally what I would like to achieve is a stellar 5/6 axis CNC machine capable of rivalling industry giants
HA! Don't we all want that?

In my opinion points 1,2 and 3 should be taken care of by the servo controllers. If you use a digital interface all Linuxcnc has to do is send the positional command.
4. VFDs, what about it? Lots of people use one for their spindle using PWM or analog.
5. Analog feedback is possible, interfaces may depend on your choice of servo interface. Although temperature sensors don't need real time, so Modbus or Profibus could be used.
6. Requires a suitable CAM solution and possibly a custom kinematic module in linuxcnc to fit your machine. So the CAM either outputs gcode that has already been calculated to fit your machine kinematics or it outputs TCP gcode that is then run through your custom kinematic model in LinuxCNC. Either way you will need a postprocessor that fits your CAM system to get what you want.

Just as general note: As can be seen in various videos on the internet, LinuxCNC is used to control a variety of non-trivial kinematic machines. I use it to run a 6 axis industrial robot. However you should be aware that there are limitations inherent in the way LinuxCNC handles non-trivial kinematics. The motion planner will fall back to a one line look ahead for anything other than 3-axis linear. Which can be problematic if the gcode contains a lot of short segments. The motion planner also has no notion of the non-trivial kinematic model since that model is added after the planning has been done. Which means it will not foresee soft limit violations prior to running the code.  So all code should really be run in a simulation first.
Another thing to note is that there are few users who will be able to advise on how to setup and run such a machine. You'll be very much off the beaten path and actually on the limits of what LinuxCNC can do.

5. Regarding Modbus and Profibus, do you have any recommendation on which way to go or any tips when identifying hardware and how to integrate it with Linuxcnc?

Modbus is used fairly often to communicate with VFD's although communication speed may be too slow for run/stop and speed commands.
As for Profibus:
forum.linuxcnc.org/10-advanced-configura...-and-profibus#222282
Note that, since this uses python code, it is not suitable for anything requiring hard real time communications.

How does one begin to rival 5 axis simultaneous cnc industry giants with this limitation then?

If you are looking for a professional solution then I would suggest you look at a controller built for that market:
cache.industry.siemens.com/dl/files/454/.../SIN_WF5_0509_en.pdf

I have no idea what something like that costs but my industrial robot was run by a controller that (10 years ago) cost 40k€. LinuxCNC is free and opensource and while certainly not a robot controller  it's the only solution I know of that let's me do anything useful with that robot without having to spend more than 1k€. 

As I said, there are users that seem to successfully produce parts on 5-axis machines with simultaneous tool paths but unfortunately feedback has been sporadic at best and none of those users seem to frequent this forum regularly. 
But maybe somebody will actually see your thread and voice an opinion.