I think the problem is you’re viewing it like you would say Diablo. Where those have acts and you can just freely roam through the whole thing.

This is d&d. So this is more of a linear story. More precise your linear story. If your story involves going from the beach to baldur’s gate and you never saw the underdark then that’s perfectly fine. There’s really no right or wrong way to play.

And that’s where the massive amount of replayability comes. Because long as you don’t keep choosing the exact same things every time you will get a different story.

The problem with this is the encounters and enemies have specific levels. If you skip things (and there are several instances where progressing one quest ends or eliminates many others) you could end up very underleveled later on. I had several fights early in Act 2 where I felt I was VERY obviously missing content as I needed my entire party to be a level or two higher to get through without save-scumming, spamming items, exploiting mechanics, or all of the above.

A lot are super not obvious too - in Act 1 there are quite a few things you're "supposed" to do before you go to the goblin camp or else you'll be pretty unprepared to deal with the sheer number of level 3~4 mobs. But I assumed initially it worked like more traditional RPGs and you do the side quests first and the main quest as the last thing before wrapping up the act. You can also sneak around and pretty much get instantly TPK'd by the Spectator until you have the action economy to deal with it (level 5ish). But in Act 2 you're actually "supposed" to go to Moonrise first before going to the Mausoleum or else you lose the ability to finish tons of quests and access to several vendors. You're told to find tieflings, Mol, and Zevlor all at the same time but you have no way of knowing that if you go to the mausoleum first you'll lose access to the tiefling quest resolution, but not the other two. It's not obvious at all that you can't find Mol until Act 3. It's missing the kind of corrections and hints a DM could do on the fly to keep things moving, or at least nudge you in the right direction if you're willing to be railroaded in order to see the most content possible.