Regardless: Trying to print with your settings from Safari gives the same result (or rather non result) as in DT. So, it’s Webkit-related, and you’d have to print from FF.
There’s Safari which doesn’t apply page-break-after although this (old?) Apple doc Supported CSS Properties says it’s supported.
There’s Firefox which supports page-break-after (or break-after?) but doesn’t support AppleScript. The first request was made 18 years ago.
And there also seems to be an old bug which Apple doesn’t care about: PDFs simply split right in the middle of an element (e.g. a header) if there’s enough content before it. Never noticed it, but now that I’m aware it really sucks.
(Anyway turns out I was looking in the wrong direction: it’s a little bit more reliable to extract the person names from the HTML source via regex instead of processing the plain text via NaturalLanguage. No idea how I managed to overlook that there’s a table like structure below the “Cast & Crew” header. Regex isn’t the best way to get the names, I know, but it’ll do it… Really need to learn HTML someday)