Sauron knowing that the Fellowship intended to destroy it

I always thought that he assumed that nobody would want to destroy such a powerful weapon. How did he learn they wanted to destroy it? He'd more likely assume the fellowship were taking it to Minas Tirith to strengthen their armies. Denethor sent Borimir to get the ring for that reason, and Denethor was being watched through the Palantir so Sauron would probably assume that was the plan all along.

But filling the entrance to the cracks of doom with solid concrete would have been a good idea anyway.