The interview process was just a series of informal conversations. The first conversation is just a bunch of trivia questions about elixir syntax and basic OTP concepts, with maybe one or two extremely basic questions about postgres. None of the questions are going to determine if you can build a production ready system and you can probably pass this interview if you just have a rudimentary exposure to elixir. There are no questions that probe or explore if you've actually applied any of this knowledge to build a system or are capable of creating business value.
The second interview was completely wild. There was no structure, it was just one person by themselves asking random questions and then drilling on your answers and how they compared to the interviewers opinions. The questions have nothing to do with your capability to do the job and everything in the conversation was entirely subjective. I really genuinely didn't enjoy talking to this person, they where incredibly pedantic, myopic, and some what unpleasant to talk to. You could tell that they legitimately thought that people who didn't know the things they knew, or thought differently than they though where stupid or unqualified. They didn't view deficiencies in their team mates as opportunities to lift them up, but demerits against their intelligence and aptitude for the job. A specific example of this was the interviewers reaction to other people who didn't know how to deal with currency programmatically. There was zero contrition that most people just need exposure to the pitfalls of dealing with currency programmatically and could quickly move beyond this short coming after a simple conversation. This tells me that the interviewer is not some one that tries to lift up and make the people they work with better engineers. This is the exact kind of person most interview pipelines seek to screen out, At one point in the conversation they asked if there where any downsides to running dialyzer. I pointed out a few things including that the errors aren't great because the technology is from erlang so they can be off putting for people who aren't used to dialyzer. They pointed out that dialyzer takes time to run as if that was some great insight and I had actually missed a real downside to using dialyzer. It's about as insightful as pointing out that lightbulbs require electricity, cars require gasoline, and grass needs sunlight. This person might do well with their day to day work, but I wouldn't be building a team based off of the kind of personalities that are going to clique with him.
My suggestion would be for the team to spend some time dog fooding its own interviewing process. Interviewing is its own skillset and the only way to get better at it is to practice. Your job advert is well written and the things you claim to be looking for are definitely the same things I would look for in a candidate, the problem is that nothing in your interview process is actually screening for the qualities you claim to be interested in. So practice interviewing each other, and if you can invite your friends and acquaintances to be interviewed and be open to the feedback and ready to adjust your process. Good luck out there.