You know, something that just occurred to me is that, since the ancient Sith were more powerful than the Empire Sith duo, why didn’t they use Palpatine’s pseudo-immortality, just with children or something? I’d expect that everyone wants a slice of that immortality pie.
I forgot what it was called, but an ancient Sith created a force technique that allowed the Spirit to life after his death and cling itself to an object or even posses another persons body. This way they could life almost forever, but they could still be destroyed. Exar Kun learned this technique, and even Darth Bane tried to use it on his pupil, but she was able to repel his attempt.
Then Palpatine learned it to switch to cloned body, which aged rapidly, and then he tried to posses the body of baby Anakin Solo, but was stopped when a Jedi got in the way, hold Palpatine’s spirit in himself and died with him.
There were a few more who used that ability, but that is all in the old EU anyway.
Also there was the Sith Emperor from the online Game The Old Republik, he created a ritual that absorbed all the life energy of an entire planet into himself, making him the most powerful and quasi immortal beieng in the galaxy.