PLEASE HELP:Replacing 1420 pages in a 29700 page document so i can import the bookmarks

PLEASE HELP:

Adobe Pro Acrobat 2017- Windows 10Pro- File size about 530MB- 29700 pages and growing. Hardware: 16 TB-RAM, i7, 1TB-HD, 64 bit

Replace (as in the Organize tool)1420 pages- (Document B) in a 29700 page (Document A) PDF Document processes close till the end per completion task bar graphic, and then Adobe stops responding when there is 2% left at the end in the task bar.

I have tried this several times but it continues to get closer to the end bar before Adobe Acrobat Pro 2017 stops responding. The full file is about 540MB and yes it is a large file.

I am using a MS Pro4 with 16GB RAM, i7, Windows 10Pro, 1TB HD. There is enough hardware but the application starts to hold back in the end, when it is stating its consolidating the formats, or whatever. I restarted the computer, still the problem persists as I several times closed the application reopened it. and tried it again and again. The problem persists.

If the application has a limitation to size then maybe I could deleted first the 1419 documents and then replace the 1 page of the document with the 1420 pages of the external document being replaced with (?).

I am doing this whole exercise because the bookmarks of the 1420 pages are in the external document-B and I found no way to import to the bookmarks into Document A of approx. 29700 pages( the 1420 pages of Document B are already in Document A but are not bookmarked like the external file is). I tried combining the files and it did bring in the bookmarks from Document B into Document A, but then IN the combined document A+B, B is at the end of the DOCUMENT A. Is there any way I could move the Document B now in Document A with all the bookmarks to the appropriate position of pages in Document A- Yes I am sure I can move them but wonder if the bookmarks will travel. Trying to move 1420 pages by selecting and with mouse seems challenging too. Anyway to move pages by stating which one and to where. Replacement option is only from an external document - is there any way to do it internally?

Sorry too many questions but simply trying to find the best way of adding documents with pre-existing bookmarks into a parent document where in after the external document is added the bookmarks continue to show up where the pages are inserted and work with the current numbering of pages .

PLEASE HELP! Ask me questions if my ability to describe the problem is a problem. Thanks for your accommodation. Cheers!