Sauron was no fool. When he was making
the One, he needed to make something that was more powerful than the elven rings. He also needed to make ti malicious. Because if he had made it powerful but not filled it with malice than an
elf or a man could have risen up against him and defeated him. And by filling it with malice it would eventually corrupt whoever possessed it and the creature would fall under his
evil shadow and so be noticed. Though I suspect he never considered that he would lose it. Especially since him losing the One was more accidental than anything else.
Isildur's stab at Sauron was more of a last-ditch stab at him because he thought death was upon him. Then in LotR All entrances to
Mordor were guarded well. He knew that any attempt to get through with a large army would be noticed outright, and a small one would be all but suicide. And if a small group did get in it would be an even tougher route to Mt.Doom.
And for the matter of placing stronger guards either at the entrance of Mordor or at Mt Doom, the strongest forces in ME that were at his disposal were the
Nazgul. He did not have any
Balrogs, other wise I think he would have had balrogs leading the forces on
Minas Tirith. And he couldn't afford to have the Nazgul guarding the borders because he knew that
Orcs were no where near as powerful as the Nazgul so if he was to stop his enemies from getting near him he should dispatch the Nazgul. Which he did. He also used the Orcs of Sauron. He had been planning on waiting before playing his hand but the finding of the One and the formation of the Fellowship made him play his hand before he had been planning.