Hello, I live in Japan. I migrated from an EU country a few years ago. I felt I should write a response outlining some experiences thus far. Bear in mind I lived in an EU country so I cannot contrast the experiences of migrating from outside the EU.
As @FranklyFlawless put it so eloquently, privacy is slowly but surely decreasing everywhere you go, so I wouldn’t hedge your bets on Japan being a last safe haven for privacy.
I understand this may not be apparent if you’ve only lived in one place, but immigrating basically anywhere means giving a lot of your personal data to the government or relevant authority. Japan’s laws dictate all foreigners carry on their person at all times a 在留カード or foreign resident’s card which shows your name, DOB, country of origin, current address etc. Police are entitled to ask you for your card, refusing to hand it over may lead to more trouble. In my experience, I’ve never been asked for my card out on the street. It also may be one of the only forms of ID you can hand over in order to avail of services like hotels, phone plans, etc.
Furthermore, Japan has recently rolled out a マイナンバーカード or MyNumber card, which also displays lots of info about you, as well as your 個人番号 or personal number, you can think of it like a social security number in the US, it’s necessary to give to employers so they can calculate your tax effectively.
The MyNumber card faced scrutiny from the Japanese public for the sheer amount of personal data it requires, but slowly and surely it is becoming more widely adopted, with health insurance cards being phased out in favour of having it stored on the MyNumber card. This means needing to bring and show your card if you want to go to the clinic or hospital. As a foreigner, I’m quite sure you are required to enroll for a MyNumber card when you register your residence at your local city hall. Anytime you move city, you have to inform your former and current city hall of the move.
As you can see, like most governments I’d imagine, Japan expects you to hand over a lot of personal info to do many things, and there isn’t much leeway. Speaking anecdotally, Japanese are relatively privacy conscious, particularly when it comes to the Internet. Most people use social media giving away as little personal info or uploading as few personal photos as possible, and people vlogging or taking photos in public places are fairly rare. Having said that, there is little reluctance in handing over info to other people or companies. I believe this stems from a belief that data leaks/breaches etc. are unlikely. Cyberattacks are on the rise in Japan as well so this attitude may slowly change.
I will say I haven’t encountered much difficulty using tools to mitigate my privacy, for example, I switched to GrapheneOS about a year ago and so far all my banking and payment apps have worked flawlessly. Furthermore, all my online accounts use aliased email addresses through things like SimpleLogin, and I have never encountered a service that would not work with the aliased info.
To wrap up, I think Japan is by no means a bad place for privacy, but I wouldn’t choose to live here purely on that merit. There are plenty of good reasons to live in Japan, though. It is a beautiful and mostly crime-free place to live. I think if I were to have children and could raise them anywhere, I would choose here. And I do not regret my decision to move.