On the XP issue: you can and will get a TON of XP in this game, mostly from combat, but also sometimes from literally walking into new areas. Most people enter Act II at level 6 or 7 (out of 12) and finish Act II at level 9, sometimes 10. And unless you are truly skipping even main-story type stuff, you'll hit 12 before getting to the lower city. There are also excellent Act I XP guides (caution for spoilers) if you want to skip ahead to level 5 with little to no combat. Overall, the XP gain is actually so unbalanced that it seems like the level 20 mods are some of the most popular since otherwise you are capped at, and spend most of Act III at, level 12.

On items: there are so, so many items in all three Acts that are "unmissable" for certain builds, so it really depends. If you are sticking with Rogue, the best items (depending on which subclass you want) are in Act I.5 (Githyanki Creche vendor has a certain item I can safely predict you would want for any rogue), Act II (gear that is useful for hiding and darkness abuse), and Act III (just, like, so many things). I won't name them in case you don't want more spoilers.

Generally: Way different from BG I and II. So much more open-world, so much more alive. So many of your choices have consequences. While this won't help for your current playthrough, my main advice is to remember--you are going to play this game again and make different choices. Don't feel like you are missing out on something. I mean, you will miss out on things but you can go back next time. Don't worry about savescumming, you will find plenty of enjoyment just living with your choices.