I'm curious, where did you learn progressive AVIF works like this? Have you actually read the spec, or does your understanding comes from somewhere/someone else and never challenged the truthfulness of it? Progressive AVIF is truly "progressive" -- it never involves "loading a thumbnail" or "layering an image over another".
In reality, each pass (up to 4) can refine previous ones (thanks to AV1's inter-encoding toolset), avoiding storing redundant information between passes. The viewing environment doesn't need to render a given pass if a subsequent one has already been downloaded. Finally, scaling is configurable -- you can have your first pass already be at full res, just at a lower quality.
Hope this helps clarify how progressive AVIF actually works under the hood.