In my opinion you should never be forced to fight underleveled. It should be possible to reach the level of the opponents if you follow the flow of the story without looking under every stone and press xp from everywhere. Who shortcuts or explores too much and faces spiders at level 2, bad luck.
We need lvl 5 before entering the Grymforge. With the current xp earn you can get to lvl 5 a good time before (spoken from experience with a mod that opens up leveling, so no guarantee how it will be in release).
In my easy-peasy uninspired no-explore rush-it playthroughs it is like this: Lvl 2 after the fight against the three misguided fishers, then the ruins, lvl 3 after the fight at the druid grove, lvl 4 after the fight against the first gnolls (after clearing the Blighted Village, the owlbear etc.). After getting to lvl 4 I fight the hag (unbelievably she wiped my party in my current playthrough the first trial, like in the very first fight against her; two of the four wipes I experienced in the whole game ever came from an old woman ...). After defeating the goblins in the temple, the Minotaurs and the Duergar in the Underdark village I got to lvl 5. That's maybe a bit too early, although it's a big help to be lvl 5 when fighting Bernard.
