Yes it was, an action to get rid of a character [superman] that Snyder couldn't handle and at the same time give Batman a redemption arc, an action that nearly ended the DCEU.
"Development"? Seriously? Snyderman wasn't even a character to begin with, he was a plot device used by a hack director. Cavill's Superman was a mopey, emo, dialogue challenged "character" in MOS, then Snyder turned him into a mopey, emo, dialogue challenged "character" with an added touch of nihilism "no stays good in this world". Then came the moody, semi-evil, "wear a black suit and don't bother explaining why" Superman in JL: the endless snyder cut.
3 (count them...THREE) movies and Superman didn't get an ounce of development, yet some suckers out there still think that Snyder wouldn't give them "the Superman they know and love" after 5 long, arduous films. Warners might've initially drank the Kool-Aid on Snyder's grand "project" and that was purely because of Nolan's involvement and backing of Snyder, but those days are over
You sure? Because that scene with Clark at the church in MOS with the red caped Jesus image behind him the whole time SCREAMED Superman = Jesus! And if that's not enough, David Goyer said that their 2 references for the movie were the comics and the bible.
Not unless your intention is to make a compelling world's finest story, which Dawn of Just***t certainly wasn't. The stark contrast between Superman and Batman is at the very heart of what makes their friendship / partnership / rivalry so appealing, 2 sides of the same coin, different means to the same ends and all that, but what the audience got was a nihilist, moody Superman and a morally bankrupt, psychotic Batman and the rest was history and so was the Snyderverse (thankfully).
Oh it goes far further than just iconography. Look up how old Clark is during Man of Steel. Do you think Snyder randomly chose the age of 33? Or that Clark has a beard during his journey to discover himself just for the sake of having a beard? Even the house numbers that are shown in the film are tied to passages from the bible. Subteltly is not one of Snyder's strenghts.
Even in your descriptions of this version of Superman, you are assigning him character traits, they may not be traits you like but they do constitute a personality. The way I see it he is a character rather than a plot device because he has motivations, internal conflict and agency.
To me Snyders Superman is be someone who is struggling with huge, world-shaping responsibilities, he is cautious and careful, wary that making a small mistake could cost countless lives. These films focus heavily on choice and consequence, Clark is repeatedly faced with difficult decisions and tries to weigh up the repercussions. He desperately wants to help people, to use his gifts openly, but is fearful that doing so might cause more harm in the long term. The idea that knowledge of his existence will change the world in a number of unpredictable ways is a weight on his mind. He wants to wait, to not reveal himself until he believes he, and the world, are ready, until it would do more good than ill. And so every time he sees someone in distress he has to make a decision, based not only on those immediately involved, but the possible wider consequences. This leads him to making a compromise with himself, he can't stand the idea of not helping, nor does he want to risk the pandoras box of exposure, so every time he uses his powers, he disappears. It means he is constantly on the move, not able to lay down roots. The knowledge that he might have to leave at any moment, and not wanting to attract attention to himself would over time train him to quiet, to be "dialogue challenged". It seems to me like a trait he would come to rely on in his life before becoming Superman.
In BVS things are a bit different, as Clark Kent he is far more talkative, challenging Perry White in meetings, Bruce Wayne at the Gala etc. But as Superman, he is still very reserved. Now that he has a public persona which allows him to use his powers, he is able to settle down and express himself more freely as Clark Kent. But as Superman he has new constraints, as a major public figure everything he says will be recorded, broadcast across the planet and analysed. He is aware of his massive influence, that he is treated as a geopolitical superpower, that some see him as a religious figure, a super-celebrity. If he makes a careless statement, if something he says is misconstrued it could change the policies of nations or inspire the actions of a fanatic. Because of this he is understandable cautious with his words, because he needs to take the same kind of care a government would with official statements.
To me at least, it appears that rather than his lack of chit chat isn't a sign that he lacks a personality, but that he has a very well thought out personality.
Similarly, I think criticising him for being downbeat is unfair, in all three films we see him happy and smiling when he's at home with his loved ones. However the nature of stories being told focuses on his trials, on him being faced with serious, difficult decisions, in such scenarios it makes sense that he isn't having a lot of laughs.
In BVS, Luthor is deliberately seeking to break his spirit, to undermine his belief in humanity and compromise his ideals. As such, he has a crisis of confidence, he goes down and the ref counts to 9 before he get back to his feet and reaffirms his belief in humanity, sacrificing himself to save them.
Snyder deliberately uses christian iconography, but I don't think that necessarily means he is uses Superman as a Jesus analogue. He also uses a lot of references to greek mythology, Arthurian legend and even the Wizard of Oz. He is using well known cultural touchstones as shorthand to try to help communicate themes to the audience. He isn't supposed to be Jesus anymore than he is meant to be Prometheus or Dorothy.
The relationship between Superman and Batman has varied a lot over the decades, sometimes they are very different, sometimes they have basically the same personality. I think there are plenty of different valid interpretations, from TDKR to Superfriends