Absolutely everything you said has merit (though lets be honest- most American's wouldn't know a British actor was playing an American character unless they A) knew of them from British cinema, and B) hear them speak with their natural accent. There have been a few actors- Florence Pugh comes to mind- that I didn't realize they were British until I hear them talk in an interview).
That being said-
For someone like me, my problem with colorblind casting is that I've spent 30+ years reading comics, where characters were drawn in a specific way. So when a comic I love is adapted to the big screen, I have an expectation that they will cast the actors to resemble their comic book counterparts as closely as possible. So if they are white in the comics, I want to see them as white on the screen.
Sometimes, I don't mind as much. I had no problem with Zoe Kravitz playing Catwoman in The Batman- there has been a black Catwoman before, and besides that Zoe nailed it. I thought that having Johnny Storm played by Michael B. Jordan in Fantastic Four was done well, in that the Storms were a African American family, with Sue being adopted by them. Probably the only good thing in that movie. I've liked Iris West, and all of the West family, on The Flash, even though the character was white in the comics. I even came around and really enjoyed Jurnee Smollet-Bell as Dinah Lance, even though I RAILED against that casting when it was announced.
But overall, I'm of the mind that if you want representation in a comic book movie, then maybe you should create a NEW character. Create something memorable, something that gets over with the people. Or adapt a lesser known or minor character, someone no one really knows. I'd say that's what they did with Dreamer on Supergirl, and she's now made the jump back to comics. They created Rene Montoya for Batman: TAS, and she made the jump to the comics and became super popular. And Marvel already changes the history of their characters all of the time to fit into the MCU- no where have they ever said Sam Wilson in the movies used to be a pimp.
I would rather them take the effort to create a new character, rather than do colorblind casting in a lazy attempt at representation.