How does the archive they provide look like? Many zip files? I would like to retrieve them and offload to another storage service but I don’t have local storage enough to hold all of it at the same time, unpack and then reupload. I would need to do it in stages.

Yes, many ZIP files. You can select the ZIP file sizes, from 1 to 25 GB, iirc. Although a few end up larger than 25 for some reason. And took 1-2 days for Apple to "prepare".

While that’s a pain for you, it’s also a pain if they have multiple files for those that have enough storage.

Photo management is a bit of a nightmare as it’s an awful lot of small(ish) files.