Speaking performance/optimisations, interesting choice to destroy canvas when it leaves viewport and recreate it upon each re-entry.

Personally I'd leave it created and stop observing the element instead, à la

    /* … */ if (entry.isIntersecting) { observer.unobserve(entry.target); /* … */ }

Do you think (or know) that swapping observed node for canvas (potentially producing many canvases) is more expensive than keeping all observers and having smallest possible amount of canvases? (Maybe I'm biased towards saving CPU but saving RAM is better after all?)