I've written a Blazor Webassembly app to edit pdf outline and metadata, available at:
https://anezih.github.io/PdfMetadataEditor/
It allows me to quickly add an outline by copying the titles from the viewer and adding entries on the left side.
While scanning books I skip some blank pages but then the toc pages does not correspond to the pdf page numbers, by setting the "Page Offset" I only follow the in-book table of contents page numbers and when the file is saved page numbers are incremented or decremented accordingly.
Source code:
https://github.com/anezih/PdfMetadataEditor