Several of the "special mechanics" are 5e already, Coating your weapon with poison is the same mechanic as dipping your weapon in fire but gives the player more versatility to overcome damage reduction (if DR is even a thing anymore). Backstab is a buffed version of flanking, during a flank with two characters vs one they would be considered flatfooted and rogues would gain sneak attack.
Height advantage, explosive barrels, etc. sure you got me on that. There's a huge thread already on height advantage and barrelmancy has been nerfed already along with basic spell's doing additional floor effects.
There is actually a big problem with 5e rules where it is way to easy to break a spell that requires concentration. There is no concentration skill nor is feats as frequent as they used to be in 3.5 or pathfinder. Which both would buff the ability to stay focused on the spell while in battle. With the lackluster focus on concentration in the rules, those spells become more of a waste of time and unnecessary feature.
It takes a full action to coat a weapon. BG3 you can dip and attack in the same turn.
The variant flanking rule requires two characters in opposite sides. In BG3 you just need to move behind them. No other character needed.
Ground effects are still plentiful with special arrows available everywhere. Special arrows already do additional damage, they don’t need to have more effects.
Concentration is a balance against OP spells. I won’t say it’s perfect but it’s not as broken as you say. Otherwise we go back to old editions where the party has half a dozen buffs before engaging.
