'page_number' and 'tumblelog' is needed to go to next page while using 'before' parameter.
'page_number' and 'tumblelog' is available at the end of the API response when using 'before' param.
NOTE; to get first page with 'before' param, 'page_number' and 'tumblelog' are optional

{
  "meta": {
    "status": 200,
    "msg": "OK"
  },
  "response": {
    "blog": {...},
    "posts": [...],
    "total_posts": 105,
    "_links": {
      "next": {
        "href": "/v2/blog/loveryuz/posts?before=<timestamp>&tumblelog=<blog name>&page_number=<random string>",
        "method": "GET",
        "query_params": {
          "before": "<timestamp>",
          "tumblelog": "<blog name>",
          "page_number": "<random string>"
        }
      }
    }
  }
}

As 'offset' param doesn't help to get next page results when using 'before' param in API call

'page_number' and 'tumblelog' missing in docs
NOTE: I'm not sure whats the use of 'tumblelog', next page results only need 'page_number' param
Docs page: https://www.tumblr.com/docs/en/api/v2#:~:text=/posts%20%E2%80%93-,Retrieve%20Published%20Posts,-Examples