In TROS we see his Sith followers. It is more likely that after ROTJ they went to the ruins of the Death Star and were able to recover Palaptine. Not sure if it was his body, force ghost, or what. He may have taken over one of them for the voyage and once on Exogul they cloned his body, but after he transferred into it the clone body could not contain him and was dying. I feel certain that what we see in TROS is a clone. We know Snoke was a clone (of who or what we don’t know) and I think the tissue samples they took from Grogu in Mandelorian are going to fit in there somewhere. My guess is that what we just saw in the season 2 finale of Mandelorian is what starts the ST. I think that Palpatine was dead for all that time and only after that was found and taken to Exogol.