I really think if you want to play an evil game, you have to design the whole game around this concept. Making a by default heroic game and trying to convert it into a game that one can also play as evil assumes too much that good and evil are the same. Which they are not.
To be honest, BG3 could have quite easily set up evil runs quite well.
If the Illithid powers were tied to evil route (Which it already has a precedent for given that the Absolutists are continually going on about granting the faithful gifts), you'd get your reason to be evil and follow the evil people, as they grant you power through the Illithid powers.
Then you can follow Astarion's evil plan of taking control of the Absolute.
Thus, the whole game doesn't have to be designed around it. Both evil and good paths follow a singular storyline (Look for ways to get rid of the tadpole > Find out about the Absolute > Deal with the 3 > Deal with Brainy McBrainface)
You just have the 2 different methods of going about it.
Evil path would get you a bunch of extra powers that are cool and useful. While the good path would get you more allies that could be helpful.
Both prepare you for the final confrontation in separate ways and you get a tangible difference in gameplay for picking one path over the other.
It's just that Larian decided to scrap any actually interesting use of the tadpoles because not enough people were using them in EA and thus we get this stupid system that has 0 downsides or consequences (Besides the icky black veins for T2 powers) being utilised by everyone who likes to min-max and is equally available regardless of the route you're on.
That's all it really takes to incorporate evil routes. Just a separate path of actions and an alternate motive for going through the same storyline. Throw in some extra goodies like bonus powers and sick loot to entice people towards evil (Also opening up morally grey heroes who opt for that extra power so they can use it for good).
It doesn't take a lot to facilitate evil and good routes in games. Which is why it's so disappointing that it's fumbled so hard. Both in terms of it hardly ever being implemented, but when it is the evil route is exclusively "Kicking puppies" and "Burning down orphanages" choices with no reason beyond "Because it's evil".
