That’s how I see it too. An artist kinda singing “it would be nice if there wasn’t suffering and killing”. And kinda taking a few jabs at some of the motivations of that. At the time nations,,,big check. At the time religion, big check with Iran, northern Ireland, etc.
I think you’re on the right track, this wasn’t deep political philosophy here.
‘Imagine’ was just Lennon expressing an empty-headed sentimentalism that sounded good to him.
He wasn’t religious so this is the sort of utopian nonsense that would pop into his head. The flaws in it are obvious to anyone who thinks about the lyrics in anything other than the superficial way that it was written.
There may be a little uneasiness with his own astounding material success mixed in as well. He probably grew up hearing the rich regarded as objects of scorn and here he was, one of them in spades.