Godzilla '54 is "nothing more than a smaller Heisei Godzilla"? How, exactly?
They have a very similar body build, they're movements are similar and they act similar. Heisei Godzilla is basically a bigger Godzilla '54.
Act similar? Its kinda hard to really sell that when Goji 54 displayed zero emotions and personality, while the Heisei Godzilla was the exact opposite.
And there movements aren't really similar, Heisei Godzilla tends to move around/react a lot more, during combat or whenever he's attacked, tends to be more involved despite looking sluggish, from biting, to lifting up and slamming down opponents, to tossing enemies, choking them, and stomping on them. Heisei Godzilla acts more lively, Godzilla 54 doesn't do that. Godzilla 54 tends to mostly act like Kiryu-Goji, standing there and taking any assault before returning fire.
Overall I kindly disagree with your opinion.