The goal is to be constructive here: Larian showed they listen, so let's make it count.

Let's be real: The end of the game is a mess, and a miss. It feels like a crash just before the finishing line.
Did the writers suddenly decide to immolate themselves after a burn out like Karlach? Did they take too many tadpoles? Did the beholder eat the script and the writers had to rewrite it while commuting in nautiloid?
Who knows. What matters is: how can it be fixed to give a proper conclusion to this otherwise memorable experience?

Here is a summary of the main issues:
- Tadpoles: so what?
Using them or not has no consequences. Neither during the game, nor in your options at the end.
During the game, the decision to take 'just one more' tadpole should have been tough, each of these bringing you closer to a mindflayer, with changes to your apparences impacting NPCs reactions. More power at the cost of greater isolation and a narrowing of your options as it gets harder to move openly and get others to cooperate.
Maybe it was too hard to implement: Fine.
But at least make it count for the endings!
No tadpoles snacks? Reward this with greater ability to resist the Netherbrain and/or an alternative option to transformation.
Someone had too many tadpoles? Makes it impossible to resist transformation and/or reduce the choices about who transforms (i.e. it has to be the character whose brain is the most corrupted).
This example highlights an overarching issue which will be my next point.
- 'A la carte' ending:
Being able to see many endings just by choosing different dialogue choices in the last sprint of the game is a sure recipe to make players feel that their previous choices didn't matter.
The end of act1 felt more satisfying in that regard.
- Sacrifice: Make it count.
We get it: sacrifice is a key theme here. Then make it count! Instead we can decide to sacrifice ourselves (to avoid loosing our personality, see another point below) without even any opportunity to say goodbye.
I would have been fine letting Karlach go and then killing myself in a Romeo and Juliette style. Instead I can just kill myself without any departing word, while I know Karlach dies...
But it doesn't stop there: Where is the meaning of sacrifice without an epilogue?
- No epilogue:
We have been immersed in a world full of lively NPCs with their own personalities and we want to see what become of them after our victory (and what we sacrificed for it!). Who care about 'the people of Baldur's gate'? It's a cold shower instead of an emotional moment.
The very least would be the narrator talking on a black screen about what became of each people we saved (or fought but is still alive) during our adventure.
- Becoming a mindflayer is trivialized
The analogy Mindflayer / Alzheimer could have been made more tragic. You loose your personality, your self.
Instead we get a 'oh you might be fine, just more tentacles'. Underplaying the consequences of choices is kind of the opposite of what good writing should be. Good stories talk about universal themes of human struggles.
- Gale: 'oh damn we forgot that option'
We have to transform someone into a mindflayer no matter what, even if we intend to let Gale sacrifice himself. We don't have the option of talking to Orpheus about this plan.
A funny bug is that even if he blows-up he still show up looking for the crown. (no one tested the various endings Larian? It took me an hour...)
Feels like last minute reshuffle for an option otherwise viable as early as act 2.
Or come up with a convincing explanation why it would not work on the netherbrain.
Or even better, let us take the risk and it doesn't work... oops.
- Withers: why?
This is a major loose end that the post-credit scene does not address successfully.
Why was he helping us? Was he acting in conjunction with other gods? The guy has been hanging in our camp for the whole game...
- Ghithyanki people arc
Sounds unfinished business too: What was Vlaakith? What were the links between her, the gods and the devils from Hell? It's clear as mud.
This could be addressed both by dialogues between Laezel and Orpheus, and/or in an epilogue explaining what truth Orpheus revealed to its people and how they reacted.

These were my reflections of the day after finishing the game yesterday evening.
Nothing on the dark urge as I didn't experienced it myself but I read the endings were disappointing too.
I'll be interested to see others' take and concrete suggestions for Larian to improve the end from a sour artificial taste to an emotion rich moment (ideally before the majority of PS5 players reach this point).