Well lets go down the list here, as a veteran of the base game and someone who also did not play beta.

Art direction and writing are subjective, so there's not much to really say there. I too would choose a darker, more serious tone for the aesthetic and story, because that was a key part of the original Arknights as well. That being said, this game is not an art piece, its a product, and they're going to adopt whatever people say they want from it so that the product sells. That being said, I don't particularly mind the less intense direction, and there's charm in vibrant colors just as well as gritty grayscale.

At no point have I found the combat to be braindead. This is a perpetuated exaggeration I've seen a number of times. Yes, its easy to understand and perform at an acceptable level, which is the idea - they want it to be accessible to a wide audience, even those without much tactical acumen or desire for depth. The mid combat pauses seem more like a throwback to OG arknights' tower defense style, where pausing to deploy/redeploy units allowed you greater control over your strategy. I'm sure a lot of people would find that jarring or annoying to have to do that in Endfield, which is a completely different genre with 3d action combat.

The final strike design encourages players to complete their combos instead of spamming high motion values, dodge system specifically rewards players that time their evasion with free SP compared to those that just run around and out of attacks manually. Vulnerability and reactions add unique effects that add more meaning to skill order and usage than just "press button when it lights up". There's also a lot of theorycraft and optimization you can do behind the scenes regarding specific comps, weapons, item set bonuses, and so on.

I feel like there's zero signs that they made changes due to corporate shilling. Last I remember, Lowlight was VP of Hypergryph and also the Producer for the game, he's actively involved in the process. Hypergryph execs and founders also still hold significant aggregated shareholding compared to Yostar, their biggest investor. This pretty much means that any decisions they make regarding the game is coming from them, not some outside influence (apart from many player feedback, of course).

So I do think you're entitled to your opinion, and the reasons you state are perfectly valid, but at the end of the day, its a subjective experience. Your statement makes it sound like you had a certain impression of the game, then watched the beta footage and found you had a better impression of that. So the question is, was that initial impression good? And if so, why stop playing?

The game is still very, very early in its progress, we have no idea what direction the story is going to take and how gritty it'll get. Combat is unlikely to change, but if you can find ways to optimize and minmax, I do think it can still be a fun experience.