As in, the problem of introducing way too many new characters all at once and expect viewers to immediately be invested in what happens to them. For me personally the movie worked reasonably well, but certainly it's a valid point - and it doesn't help that the credits scenes are introducing even more new characters, when so many of the existing ones have had so little character development yet.

Also, the character's names are spelled Ikaris and Sersi, respectively - in fact, until reading your review, I hadn't even thought of the Sersi-Circe link (in my native language, we pronounce Circe in a way much closer to the original Greek, so not like 'Sersi' at all). And while I did obviously notice the Icarus/Ikaris thing, I'd actually managed to miss the whole flying into the sun thing... that one, I have no excuse for.

The similarities don't go all that far beyond the flying part, true - but then, the same goes for the others. Thena's character doesn't bear much resemblance to that of the Greek goddess Athena, either, other than both being female warriors. They aren't supposed to be very close parallels.

I interpreted it as a combination of guilt for his betrayal of his orders, and simply not seeing any path forward as he can neither go back to Arishem, nor openly fight against him. But it's certainly left unusually vague, considering how radical a move it is. As you said, he's had probably the most character development out of the ten Eternals, and then he just goes and kills himself? Though I do wonder if that's actually the end of him - I wouldn't be surprised to see Ikaris again in an Eternals sequel, whether he actually survived that or is reborn somehow.

The plow thing was silly, yes - at best, it might have been a more efficient design for plows, not the invention of the whole concept. On the other points, it seems like generally Arishem has a pretty hands-off, big picture approach, thinking in a timescale of billions of years, so humanity needing some thousands of years to evolve isn't any particular concern. Of course, Thanos' snap that killed 50 percent of all beings in the universe, not just on earth, would have been a different matter, which indeed you'd think should have concerned Arishem more...

Yeah, that was a wasted opportunity, alright.

I wouldn't exactly say no attempt is made to explore that, or that Ikaris' position isn't given any credit at all - they do have some debate about it, but it's very brief, yes. As for the other thing, the idea is that after the last Deviants are - supposedly - destroyed at the fall of Tenochtitlan in the early 16th century, the Eternals go their separate ways and are generally allowed to do as they please, even interfering with humans, presumably within some boundaries but very wide ones - like, if Druig had started mind-controlling half the planet, they'd have interfered, but his village in the Amazon, they let slide. I don't really see anything especially illogical or immoral in that, given what their mission is (or at least, what most of them think their mission is).

I liked it well enough, in some ways more than e.g. most of the Iron Man movies, but definitely it has issues and probably should have been handled better. Perhaps by having a few Eternals show up in a different Marvel movie to help reduce the amount of exposition needed in this one, or maybe an Eternals TV series first before the movie. Or just a different approach to writing this movie.