Feeling very depressed

Really sorry to hear about all this. Normally I recommend that you don't quit to interview prep, but it seems like your situation is really bad (is there not a way you can switch teams internally?).

Failing 9 onsites is rough, but that's how the market is right now. I know folks who have failed 10, 15, and even 20 onsites. In these situations, all you can do is get back up, dust yourself off, and try to learn from the experience (there are rarely interviews where your rejection was 100% the fault of the interviewer).

Keep in mind that hiring managers and recruiters discriminate heavily against employment gaps, especially now. I would try to stick it out at your current company as long as possible, putting in minimal effort to get by and mentally compartmentalizing away the work experience as much as possible.

Lastly, if you have a non-terrible manager, I would talk about this with them as well. Maybe they can put in some mitigation measures to boost your mental health a little bit.

Best of luck!

I feel for you and I totally think your feelings are valid. Taro is the perfect place to brainstorm solutions for this.

I have a few suggestions that helped me when I was on a similar boat.

1. This overarching feeling that you are better than and bigger than any job you will ever have or any company you work with.
2. Having a friend at work! If there's at least one other person in your position at exactly your company, it makes total sense to confide in them.
3. Sometimes being entirely high integrity with a pool of manipulative people does not help. You seem like someone who likes to put in the hard work and help people. Here's an article I have read a few times in situations like this - https://bayareacbtcenter.com/top-10-manipulation-tactics-and-how-to-counter-them/
4. The fact that you got 9 calls is AMAZING! In this economy, in your circumstances. I would be singularly happy just based off of that. What say, we apply for jobs after getting a few more LCs + system design prompts + behavioral mocks in? I am sure you'd get 2 digit call backs and probably a few offers too.
5. One last appeal/suggestion - Imagine a time in the future when you build teams/a company. Now that you've been thru this experience, you know exactly how company culture should NOT look! You know who you should NOT hire and you know where and how toxicity seeps in. These learnings are vital! Leaders like you are indispensable in weeding out toxic cultures, so onwards and upwards!

You got final round interviews with 9 companies -- that's a huge accomplishment.

If you're miserable with your current job and your work environment, you should quit. Your track record indicates that you can land something good.

Were there any patterns you recognized from the interviews you did? Was there an area you were consistently missing?