I think Ikaris and Sersi only split up ( he left) 100 years before present day. So they were together
for a very long time.
I had a very similar take, that he bottled up his emotions - and I too found his breakdown at the end had a lot more impact because of that. I wouldn't call him wooden ( as in Hayden Christensen wooden) but he was probably a bit too restrained to charm audiences.
Also, some of the present day scenes where he's a bit aloof, he's also grappling with the guilt of killing Ajak - so thats wearing away at him during his interactions with the others e.g. when he tells Kingo, who nearly worships him as a hero, "I'm not who you think I am".
He's caught in an impossible situation between Sersi and the other Eternals, and his duty to the Celestials ( and arguably the greater good). Because he and Ajak know about the Emergence, and that they've done the same thing many times before he just sees us as one more species that's reached the end of our cycle, and now it's time to start again.
Guess it didn't work for some folks , but Ikaris was my favorite character by far.