Sutta, logic.
If anything is leftover, given that parinibbāna is permanent happiness, whatever is leftover could very well be taken as the true self.
Even if say someone really consciously don’t think of it as self, nevertheless, one has to be careful about subtle delusions which underpin all of these.
Just read all the arguments that says nibbāna is cessation.
I don’t see how one can go beyond nibbāna. See from perception of the formless realm of nothingness, only nothing remains. And then neither perception nor non perception, then cessation of perception and feeling. With nothing left to cease, for the arahant after death, what is beyond?