There were really only two other suspects- The stranger (who is much more like Gandalf/Istari than Sauron) and the creepy Eminiem creature. The issue with the latter is that it had virtually no development.
I think the impact would have been better if Halbrand was a charismatic and deeply likable character, and there was a deceptive romantic relationship between him and Galadriel already. So the fanbase would be like "HalbrandGaladriel" stans and so forth. They didn't pull it off though as both Galadriel and Halbrand are unlikeable.
Sauron, according to Tolkein's lore was a shapeshifter who was charismatic and likeable but at the same time had a 'tinge' of creepiness/weirdness. I guess they tried to capture this in the show in their own way but not successfully.