Loot is one thing, but for me the reward is in the outcome of the plot and the immediate world around me based upon my decisions and thus my morale compass.
So far the evil route doesn’t appear to sell itself at all to you. Even the dream sequences don’t seem to suggest control and power, at least not enough to overwrite the idea of it being a horrible trap into giving in and becoming a mindflayer.
Then no one else “I met”, even suggested another way. Again, the whole set-up is “get rid of tadpole”! Every character we meet who tags along says the same, Raphael tries to tempt us in order to help. That is for all intents and purposes “our story”.
So based upon that how do I act? And I think this is the issue of kinda not being that important (hero/messiah complex) and being free to waltz around. No one tries to pick you out of a crowd. Anyone you meet usually ends up wanting to insult and or kill you. I certainly feel as though the goblins need to not just be subjugated by you using your tadpole but also actively then want to get you in front of Minthara first and foremost. Then it’s a who makes the best case. I might not trust Minthara, but if she sells the Absolute and being able to help me sooner then I might listen.
The issue is, by the time I get to her I might as well just slaughter them all and free the Druid. There you go, I’m either a hero or still a murderhobo but it’s disappointing that I’ve not been sold a seedy darker route out of my predicament early enough for me to care.
I caveat all of this by saying I’ve only done 1 play through so far and was rather chaotic good/flirted with evil, so I’m not a character to seek it out, but that has always been my point, I shouldn’t have to work at being evil, evil should be the easy route. Avoiding the temptation, working for a good outcome and becoming the hero, that’s the challenge and that’s reward.
TLDR reward very much = story (oh and the odd shiny item).