I like the idea of Sauron hitting rock bottom. I assume he will be more or less in solitude (with the exception of the ghost of Thuringwethil), kind of like that of a monk or hermit, but as you suggest, he’ll grow into a dangerous fanatic terrorist or prophet. He will perhaps develop extreme self-criticism, later on leading to the repentance. (I think he should actually repent, but then he experiences a shocking revelation, as if struck down by an invisible enemy, and realise he has to fulfill his Lord Melkor’s plans for the world. The moment has to be truly shocking, even for the viewers.) He will surely ultimately develop ideas of discipline and organisation, go all in Sun Tzu and Machiavelli, turning Middle-Earth into his own chessboard, but that would come later, in the Second Age. Now, at the bottom, he should dwell in a theoretical and logical ground zero, pondering ideas about purity and death.