It's just the way Portman rather thumbed her nose at the films and the genre. Hell, she couldn't even be bothered to film some new stuff for Endgame (for whatever reason). Now here she is posing at Comicon looking, IMHO, "make nice & smile" and totally unenthusiastic, with a look in her eye that seems to say "Thank Christ they're paying me a boatload of cash or I else wouldn't come near this insultingly mindless fanboy garbage with a 30meter cattle prod." I'll give you V for Vendetta, (the only movie I've seen that she was in where I liked her performance). Haven't seen or even heard of Black Swan.
The all-girls kickin' A scene in Endgame just made me roll my eyes. It was tolerable in Infinity War but it was just "Hey! We're representin' it!" the second time around. I heard more than a few groans around me in the theater but my wife liked it, so I suppose it did its intended job. And yes, Carol Danvers appeared to have been shoehorned into Endgame just... because.
For Ragnarok, it wasn't so much Thor's attitude (although he seems to get stupider in each successive film). It was the whole way the storyline played out. Thor's best friends all die and... nothing. Asgard blows up and it's citizens are all like "OK, well THAT was decidedly inconvenient. How disappointing. Oh well, does anyone have the time? It must be around 6 o'clock, because I'm hungry."

Yes, I would love to see Branaugh return. He gave the characters the resonance and air of theatricality they needed to feel just slightly out of step with us on Earth. By the time we got to Ragnarok, the Asgardians all acted like a bunch of bored losers wandering around at a Renaissance festival. Goldblum would've been a fantastic fit in one of the GotG films, but in Ragnaok he was just an irritant, like a harmless and really annoying gameshow host.
Your solution for two Thors makes sense. Still would like to see BRB. Perhaps someday.