Commercial REBCO tape is an entirely sufficient superconductor for tokamaks. At this point the limiting factor for the magnetic field is the structural strength of the reactor. Tokamak output scales with the square of size and the fourth power of magnetic field strength, and using REBCO, the CFS ARC design should get practical power output from a reactor much smaller than ITER.
About the size of JET. It's definitely practical in the sense that we can build it and it's likely to produce overall net power. Whether it will be competitive is another issue, and for that I agree with you that other designs, like Helion, have a better shot.
That's not a definition of "practical" that I would use. "Possible", perhaps, but practical implies effectiveness and suitability, and without competitiveness that isn't there.
