Just another nudge in the hopes someone else has had this issue and managed to get around it.
What I've done so far
Wait to let it unpack (essentially been letting it do this as much as possible since release, and it hasn't fixed it)
Delete/uninstall the game and redownload several times
Change servers Steam is downloading from
Close and restart Steam several times.
Empty Steams download cache
Disabled all the various DLC's/Extra's
Leave it to wait - at first it IS unpacking, and spends around an hour or so doing so. I can see the files being unpacked in the "download" section of the steam folders (and leaving the main folder where the game presumably ends up empty) but then it just comes to a halt and steam shows nothing more happening, nor can I see anything happen. For reference, the "PoE2 data file" stops unpacking at 2532 files, if that might indicate it has stopped before it should perhaps?
Strangely, if I click on the exe file, the game does start at least. But if I try to click on any of the the preorder/extra stuff it all claims that is corrupted and can't be opened. Nor does it recognise I have any of the DLC's and I am hesitant to run the game from this, if it still believes it needs to download a patch or does need to unpack more.
If Steam refuses to do anything, does anything think it would be at all possible to see if I could contact support to get a GOG key instead to attempt that? I know it said the key generated would be final, but given I currently can't even get the installed, let alone play, it is rather frustrating.
Thanks to anyone who can help!