Addressing your first thought…anything that you would call “objective” can be “doubted” by ceding the tiny tiny possibility that you are a simulation or Boltzmann brain or brain in a vat. The evidence before you may not actually be representative of the “objective” reality.

The fact that there is experience at all, the contents of which may be “doubted”, cannot be doubted.

I’m not unequivocally claiming this but that’s the thrust of the argument.