As with a lot of places, the situation in Japan is complicated and there is a degree of nuance. As a foreigner, you only need to change the gender marker on your passport (which you can do before or prior, although it would be smoother prior), after that you can change all of your documentation in Japan to reflect your gender, including Alien Residence Card and Health Insurance Cards.

The more strict regulations, surgery and sterilization in order to change your gender marker on official documents are only required of Japanese citizens. Admittedly, that is not great, but it has been undergoing increasingly intense scrutiny.

There are a number of resources for getting trans healthcare in Japan. I think if you search for "G-pit GIDclinic" There is a fairly comprehensive list of clinics that provide HRT and trans healthcare. Stonewall Japan also has a good guide for transitioning in Japan as well

As others have said, people are really kind and polite regardless. Support for LGBT issues is steadily growing across the country, albeit not yet at the highest levels of government. I think some statistics put like 80% of Japanese people in favor of Same-sex partnerships.