Best prepare for release day? Be prepared to be patient. There's a lot of speculation that Steam and GoG will be smashed, and that speculation is pretty reasonable.
Getting a 5e Player's Handbook isn't a bad idea. Even with all the changes, I've found it helpful to have a quick reference to class and spell descriptions in something that's indexed, rather than needing to google. And it's a PHB, and they're always enjoyable to flip through.
I'd also plan on starting over a few times. Larian have tried to address this with a respec-is-cheap-and-easy mechanic, and this is the first time I've given that advice since that was announced, so I don't know if it's relevant. But if that mechanic didn't exist, I would plan on two or three false starts before you land on something you want to commit to.
In terms of feeling way behind, don't worry, we all are. Even those of us with hundreds, and in some cases thousands, of hours in early access have learned about so many changes in the last week that we'll likely feel very much like we're starting fresh when we kick off 1.0.
In the new community update, Larian recommend *not* joining EA now in the hope of getting a little progress in first, because save files won't carry over, they would only recommend you do it for the "Digital Deluxe" upgrade. I disagree. If you join EA with only weeks to go, and you do with the knowledge your saves won't carry over and everything will be significantly different, I'd reckon it's worth it to try out a few classes, get familiar with interface, etc. Yes, there will be changes to all that stuff, but if you feel nervous about settling in to games of this size, a few weeks of throw-away time might be worthwhile, and I promise you they'll be fun and immersive.
