I think Sam wouldn’t have the conviction to get things done. He wasn’t the one who stood up and accepted the ring at the council. Sam was loyal and didn’t have lofty desires, but he didn’t have the spirit of adventure and perseverance that Frodo had. He was the perfect ally to help Frodo, but he wouldn’t have made a good Ring bearer himself.

Not entirely. Sam was tempted, and if he possessed the ring long enough he would have been overcome like any other, but his Hobbit-sense saved him in that one small moment:

"“As he stood there, even though the Ring was not on him but hanging by its chain about his neck, he felt himself enlarged, as if he were robed in a huge distorted shadow of himself, and vast and ominous threat halted upon the walls of Mordor…”

"Wild fantasies arose in his mind; and he saw Samwise the Strong, Hero of the Age, striding with a flaming sword across the darkened land, and armies flocking to his call as he marched to the overthrow of Barad-dur… He had only to put on the Ring and claim it for his own, and all this could be. "

"In that hour of trial it was the love of his master that helped most to hold him firm; but also deep down in him lived still unconquered his plain hobbit-sense: he knew in the core of his heart that he was not large enough to bear such a burden, even if such visions were not a mere cheat to betray him. The one small garden of a free gardener was all his need and due, not a garden swollen to a realm; his own hands to use, not the hands of others to command. "