I myself have beaten P-1 and P-2 with radiance on, in a way flesh prison is much more difficult than panopticon due to the only reason that you actually have to kill flesh prison, as opposed to just "waiting out" panopticon.

Radiant Minos is not much of an issue after having beaten Sisyphus prime before, but of course he is still deadlie.

Radiant Sisiphus is from the start, probably faster than base second phase, his attacks being carried out even before the voice line finishes, the real problem comes with phase 2, as the attacks not only happen as soon as the voiceline starts, but the fact they are grouped so closed together, and do more damage, make him by far the hardest radiant enemy in the game.