The endings are awful.
As stated, there are dedicated threads to just about every variation of ending. I can't think of an ending that people actually like and I personally haven't liked any of the endings I've seen (over 400 hours, seen 3 different endings).
Here's how my first ending went as it was somewhat similar to yours and this was the first "dagger in the heart" moment for me with this game:
After a lackluster Act 3 with little to no reactions from companions, the thought was "well this is rushed, but maybe they stick the landing with the ending."
Couldn't have been more wrong.
I went with a custom Tav. She was a drow sorcerer that romanced Astarion.
I LOVE Astarion. He's the best, most well-written character in any game I've played. I know you played his origin but I highly reccomend trying him as a regular companion because you won't get anyone more cheeky, sassy, complex, emotional, or with depth of character.
I was completely enthralled and excited to see what ending they had cooked up for us.
I convinced him not to ascend, like you did, and we got to the docks after beating the elder brain.
I was waiting for an ending moment with him. Something poignant and meaningful and some sort of end romance scene to cap off the experience.
No. Nothing. He burns up, runs off, and my character just sits there dumbfounded while one of the other companions, in my case Wyll, says a snarky and annoying comment about him "remaining in the shadows" or some shit.
So basically, I just watched the love of life who redeemed himself in every way to the others at this point, stumble and fall in a comical fashion as he burns up, and my Tav, what, doesn't care?? Doesn't run after him?? Doesn't even wrap a cloak around him?? Nothing?!
Not only that but, while I was able to convince Karlach to go to Avernus, her ending was only one who got any attention. I wanted Gale to get the Crown and his ending was just "Okay. I'll do that. I'll become a god." And then he walks off...
Then, Shadowheart is there, my second favorite character, and I was interested in what she was going to say. Guess what? Same as you. Nothing. She says literally nothing.
In addition, I learn nothing else about any of the other companions, learn nothing about my impact on anything, no final cutscene or party. It was absolutely the most unsatisfying ending to any game I've played.
The only solace was, I did get a final conversation with Astarion (you know, after finding out through no effort on my character's part that he didn't, ya know, DIE) but even then, you and Astarion stand ice cold, 10 feet apart, show absolutely no affection while he basically reaffirms "Do you still like me??? You sure??" And that was it.
Complete and utter bullshit at every turn. I hate that you had to enter the endings without a forewarning. It really is, as you stated, a slap in the face.