One thing to consider is adding something like cargo’s cargo:warning=MESSAGE, which allows build scripts to show warnings to the user even if the output is otherwise hidden.
A lot of modern build tools (e.g. ninja and cargo) show single line progress bars, but I don’t know how easy it is to extract that information and pass it back to pip. For cargo at least it involves parsing json and counting yourself, while for ninja I don’t know about any structure progress output. Again I think writing a specially formatted message to stdout/stderr could be a nice way to communicate that.
I also love the docker buildx output, even though I’m not sure if that much should be shown by default or only with -v.