I just tried out this possible solution and it didn’t seem to make a difference unfortunately. I also replaced the banner image with a trivially small image as a sanity check and it also made no difference to LCP. Thank you for the suggestion though
I don’t know the inner workings of how theme components get injected into the page but my impression is that by the time the banner component is injected, it’s already too late. My next attempt will be to try it as a plugin.