Right now I'm having to copy the commentary manually, and filling in the [tn]Source:[/tn] blocks as well.

Example:

• post #5994576
• https://www.pixiv.net/en/artworks/104214277
• https://www.pixiv.net/requests/122459

Pixiv doesn't use a JSON endpoint for this, but the data is available as JSON encoded into the page itself.

• HTML ID: __NEXT_DATA__
• JSON path: $.props.pageProps.serverSerializedPreloadedState
  • This is an escaped string which is also JSON, i.e. the " are \" and the \ are \\
• Sub-JSON path: $.pageCommission.requests.122459.requestProposal.requestOriginalProposal

It's also available on the <meta> twitter:description and og:description in the page HTML , although I don't know if those would potentially strip/crop information. Using the JSON I described above seems like it would be the best way to get the original data.