Generally, I think it has been a day or two before units start arriving. I'd guess press get their take-home demo units at the even and a set embargo time for their story. But who knows when this time, the ordering right after the event instead of the following Friday was a surprise switch.
On your memory question, system-wide functions are not going to hit 4 vs 6. Apps may, but just like in computers they are going to have to be heavy hitting apps to make a difference. CAD programs, heavy Photoshop users, etc. or heavy hitting games would likely be the only culprits.
Quite a jump in price, nearly half the ipad itself going from your 256 to 1 TB, just to future proof. Stick the $600 in an interest-earning account for a year or two or however long you need to wait, then add some money and put with it. You will be MUCH happier at your next upgrade rather than holding onto a 2GB extra memory 2018 pro.
tldr; Get the 1 TB and 2 extra GB memory if you are going to be using that soon, otherwise save your money as an additional $600 isn't worth it for future proofing.