If he had he would be a lame duck from the outset.

This is a bone headed take. A lame duck is not a lame duck simply because they're not running again. They're a lame duck because they're unpopular and politically irrelevant. If he had declared to be a one-term president, he could have spent his term building up a bench of potential candidates for 2024. He still would have had great power, influence, and the bully pulpit, even if he wasn't running again. He wouldn't be an individual, he would be part of a team. And that team is what would allow continuity and the prevention of a lame duck term.