Moving in
Export your workspace as Markdown, drop the zip in, and your pages arrive with their nesting intact. You pick which pages come across before anything is written to your vault.
In Notion, open the workspace settings, choose Export all workspace content, and pick Markdown & CSV as the format. Turn on Include subpages so the hierarchy comes with it. Notion emails you a zip, usually within a few minutes.
In Sumibako, open Settings and choose Import. You can drop the zip straight onto the panel - there is no need to unzip it first, and zips inside the zip are unpacked for you.
The importer reads the archive and shows you the page tree it found, with a character count beside each page. Nothing is written to your vault until you confirm, so an export full of pages you no longer want does not have to arrive in full.
Pages are created in batches with their nesting preserved, and you land in your vault with the tree already built. Everything from one archive is tagged as a single import, so it can be undone as one thing rather than page by page.
Nothing is written until you have seen the page tree and chosen what to keep. If it is not what you wanted, the whole import can be undone in one step.