There are a lot of things to consider before going the route of abandoning randomness. I hope this post will give a better understanding of how combat works. Currently in-game it's barely shown how modifiers impact the game and it is important to know that modifiers impact everything.
1) Most importantly the game does not just use a d20 to decide outcomes. Most cases you need to roll to beat an opponents AC (armor class) and you get the benefit of proficiency and modifiers "If (1d20 + Proficiency + Modifier) >= AC, the attack hits". Right now Baldur's Gate 3 is balanced in a way that expects you to min/max (invest in the stats the benefit the class the most). I've tested this a few times and it matters a lot, I've had a lot more success with a +3 modifier instead of a +2.
It's also important to know when you're rolling advantage "If (Max(2d20) + Proficiency + Modifier) >= AC, the attack hits" and when you're rolling at disadvantage "If (Min(2d20) + Proficiency + Modifier) >= AC, the attack hits".
Baldur's Gate 3 currently has arbitrary moments where your character can gain advantage or disadvantage. It's easy to abuse these mechanics and a lot of folks on the forums want them removed so that combat can become more engaging.
---> NOT IN DND 5e [Advantage (backstab, higher ground) Disadvantage (threatened)]
That is correct, and I'm aware of those - that is why in my post I said that calculating your hit chance ( attack roll ) does not change at all . All it changes is that after you calculate that player has 70% chance to hit, you do not random for hit/miss but you instead reduce damage by multiplying it with 70%.
To be more clear, game already change your hit chance ( shown when you hover over target ) as you change conditions like gain advantage by height or counter disadvantage by casting light on shadowed enemy etc. What will change in my suggestion ( when that option is ON ) is that game will change your expected damage, so it will show for example "3-4" damage without advantage and will change to for example "6-7" damage if you gain advantage - instead of showing hit chance percentage on hover, it will show this expected damage, while still showing reasons for advantage/disadvantage in lower left corner.
