From observation, a lot of points of contention would be eased if people forefront the genre of the game they're playing and that it's a game in the first place.
This is a mobile game with the aim of making money. This is not an e-manga, or a (buy it once) visual novel, it's a format that is aimed at getting you to come back for more, and hopefully spend money on in-game purchases.
This is an idle game with a monthly story update, with the story being its main selling point - this means the game needs to keep you interested in coming back each month, so it makes sense for the writers to set up mysteries, cliffhangers, and include frustrating half sentences with implied lore-drops, because you need to want to come back next month and see where things are going. This isn't to say we can't or shouldn't criticize the writing choices, because there are definitely a lot of things to say, just to say that this particular thing is baked into its design.
This is a joseimuke - the game is designed around characters being appealing to varying demographics. Characters are designed around an archetype principle to appeal to different types of people, and their dialogue in the main story is designed to be read as potentially romantic (to those who enjoy the character) and otherwise (to those who do not care), therefore, people who enjoy character X will interpret their actions more romantically than those who aren't, and people who hate character X might find their actions creepy, annoying, or otherwise not enjoyable. So...
Self-shipping / oc x canon is expected because of the above point, but these are fictional characters, not real people, and it's important to remember that people typically have 2 reasons for stating their dislike of a character: 1. The character is not appealing to them, and therefore it's a personal preference, 2. They are criticizing the character writing (which is still personal preference, but from the perspective that these are fictional characters in a fictional story). Your version of character X exists in your head, and is based on stuff in the story, and no matter how "correct" your version of character X is, you will never be able to police other people's versions, just as they can't do the same to your version.
The game has lots of issues, but again, its main drawing point is the story, which people generally enjoy and come back for (which is why we are here). I personally think the game play is a bit forced and is just there as an excuse to have a gacha system, but that's a confession for another time.
The MC is not her own character. Because of the game genre, the MC, like MCs in other similar games, is supposed to be a "blank slate" stand-in for yourself or an OC, and you're kind of meant to zone her out while reading, hence why the dialogue options are slightly open-ended and there isn't much implication of hobbies, strong personality traits, etc. Of course, a petite woman with pale skin and brown hair is probably not a majority of the readers / players, and we can have a conversation about how that's usually the default design and why (and how games can get around it). But then, also, why are we bringing up the MC not having any personality and always falling into terrible situations? No personality = you're meant to project, terrible situations = it's the appeal of a joseimuke game with a dangerous setting, the characters saving the MC is a majority of the romantic bait.
All the characters are somewhat off-putting, the English game description on the play store is literally "...your unruly classmates have sold their souls to demons in exchange for a wish… and you’ll have to find a way to make them work together to solve supernatural cases all over Tokyo." Part of the game's selling point is that the characters are a bit fucked up, but when you enjoy a character you see their other sides, for example, Kaito is socially awkward and fumbles flirting with the MC, and he lacks in confidence: if you like Kaito, him saying stuff like "You're going to be all alone with another guy?" is read as him being jealous in a cute way, but if you don't like Kaito, this can read as him overstepping boundaries. Same with everyone else, in their own ways.
When engaging with "hate posts" aimed at your favourite character, detach yourself from the blorbo mindset and remember other people have their own preferences / you can criticize the story and writing while still enjoying the game.
The overall way people treat writers is upsetting: people will write your favourite character "wrong", people will avoid writing your favourite character altogether, people will write "badly", but if you keep being entitled and make posts like "urgh everyone writes like shit" no one is going to write at all. Fandom writers are engaging with a hobby and sharing their work for free, they are not getting paid, they are not doing it to get paid in the first place, they are just having fun.
Following that, you can do whatever you want. You can close tumblr and write self-ship stuff in a notebook, you can make picrews of your OC, you can draw your favorite character doing a backflip, you can post your own fics, but just be kind to people. The world is not going to end if randomstranger123 headcanons that Taiga never showers, and the game isn't going to stop existing if anotherstranger456 says they hate Yuri with a burning passion (examples are random). Tumblr or any other fandom space is there for people to have fun and share their love of a common piece of media, it isn't there for people to with-hold The Correct Opinion (tm). Start to enjoy having discussions with people that think differently from you, and be respectful of other people in the space, otherwise no one will even wanna be here in the first place.