One thing you could try is search on china websites for someone selling that chip (aliexpress etc)
Then message the seller and ask how much to buy the full datasheet for said chip.
The one thing people in china like to do is make money
Even if the seller doesn't have the full DS themselves they probably know someone who knows where a copy is stored on the net.
Get them to send you the first 5 pages to prove they are legit before sending them any money.
Pro Tip:
Sometimes the hardest part of finding a datasheet on the internet is finding what the official filename is.
Once you have that finding the file is usually pretty easy.
I used to love those sites that hosted datasheets for a download fee.
They would let you see the full filename and it was the click to download that was paywalled.