You are forgetting one little fact. Most of the dnd 5e campaigns are adapted for low magic. This means that a player should have zero or only a few magic items. Of course, this approach is not a very good idea in the case of a fairly long game and players themselves love to get upgrades (colored items in the games did not come out of nowhere). So we will most likely be much stronger than the typical group of a similar level. I wouldn't worry about the clash design either. Whatever the case, larian knows how to design them (although there are some glitches like the black pits in dos2) and probably the game will not be as unfair as Owlcat games.
"one little fact"? "Most of the dnd 5e campaigns are adapted for low magic"?
I don't know if I can agree with that as being a fact.
Take Descent into Avernus. They give you a pretty nice dang magic flail almost immediately that my players gave to their dwarf fighter which did some hefty damage throughout the campaign. They also drop numerous scrolls and spellbooks for mages right in the first chapter. You go up against devils and such before Chapter 1 is completed, and I'd say it's quite full of magic. I certainly wouldn't call it "adapted for low magic."
Anyway, if I was DM'ing BG3, here is how I'd build encounters:
Prologue - Initial fights - 3 Lemures, dropped onto the nautiloid by imps who are flying around fighting intellect devourers and mind flayers and such leaving you and your baby level 1 party to fight the baby devil monsters. Lemures have 13 HP and 7 AC with a +3 to hit with their fists, and they only do 1d4 damage. They only have Cold Resistance and are immune to fire and poison. 4 Lemures would be a perfect first fight for newby D&D players and newby players to BG3. They could get their feet wet against blobbish mucky horror humanoid-ish creatures that are killed with a couple of hits regardless of your class.
Prologue - Optional fights - IF they throw intellect devourers at you, they should be called Ustilagor which are baby intellect devourers. Us should be one of them. They should be just like normal intellect devourers with maybe 11 HP, 12 AC, no Resistance, +4 to Hit, 1d4+2 damage (instead of 2d4+2), and only one attack per round. So, basically what Larian did give us except they call them Ustilagor so we know they aren't adult intellect devourers. They can't devour intellect or body thief yet because they're babies. Also, they don't do as much damage which is very important at level 1 especially for baby classes like wizard.
Prologue - Helm fight - 2 Lemures and 1 Hellsboar in the first section and 2 Lemures and 1 Imp in the second section or something similar with the imp being sort of a final Prologue boss fight. A single imp is a tough enemy for level 1 characters. It should have all normal resistances and abilities, making it a good final boss of the prologue. Whether you attack it with a party of 4 customs, Lae'zel and Shadowheart (and maybe Us), or it is just you and Lae'zel and Shadowheart... whatever... the fight should be appropriate. Keep in mind, the poison sting of an imp does 1d4+3 damage, and if you fail the Constitution save of 11 you take another 3d6 poison damage. That's a nasty end boss fight. 1 Imp is more than enough. And if you recognize you're totally outclassed and gonna get your butt handed to you, just run for the helm and don't try to kill the imp.
Beach Nautiloid fight - 3 Ustilagor (as described above) or 3 thralls using Bandit stats with only daggers would work just fine. Don't be throwing 3 full blown (even if they are injured) intellect devourers at us and then strip them of all their basic Resistances and abilities.
THAT is how you build true D&D 5e campaigns using proper stats and with the proper rules. If they did this, they'd be able to make Normal Difficulty as true D&D 5e rules with true D&D 5e monster stats and it would not be too difficult. It would also give them more room to BUILD UP to tougher monsters that require magic items and spells. By the time they reached the Underdark, THAT is when acquiring magic items and stuff would be beneficial as they face the bulette, minotaurs, a TRUE spectator, drow, etc.
