LotR Sauron vs. Silm Sauron?

[Inziladun]

Nogrod

Is it just that these are written afterwards and the Sauron of the LotR is just the immature and incomplete version of the Sauron to whom Tolkien finally had time to invest and think about later?

I think it important to keep in mind that the events in LOTR were ostensibly recorded by hobbit witnesses. When did they have an opportunity to observe Sauron in person, to get inside his mind, to explore his motivations? He spent the entire time in Mordor, and had no personal contact with any of the principal actors of the story. Who would have been in a position to report details of his personality?
During the First Age, Sauron, as the prime lieutenant of Morgoth, was obligated to personally oversee the accomplishment of many of his master's objectives. This would have brought him into more personal contact with his enemies, and thus into the written histories as something more than a far off, generic evil.
In the Second Age we see even more of him, again, because he is personally going about achieving his own ends, and not leaving all to subordinates.

Gollum the Great

Well, I wouldn't say that LOTR Sauron was personality-less. I see him more as an evil mastermind/lord/Satan bereaved of his superweapon to conquer the world in one swift blow. After failing to locate and reclaim his weapon he simply does the smart thing: maneuver what other forces he has into position so that no one else can rival him with his ring.

Sauron had, in effect, become like Morgoth: content to trust his servants and simply direct them to do his will. And really, what need was there for him to do otherwise?