Kind of limited due to there not being an iOS version, but Briar is pretty decent. It was made to be usable in repressive areas by press and other groups, as well as in areas where bad weather has taken out cell and regular wifi. Can be used with phone data, but also offline via ad-hoc wifi and bluetooth. But stuff like Signal and SimpleX are more overall useful to more people (and I think SimpleX also supports offline local immediate area of each other like wifi and bluetooth but I don’t remember atm).
What are they doing these days that is invasive (I have been wanting to go back to visit friends)?
The only time I have visited Japan was back in 2012 and all I remember was just waiting in a line and handing over my passport to a customs/foreign visitors person. I also might have had a paper slip with my dates of arrival and departure, that I wasn’t bringing in more than $9999.99 in cash, and the address I was officially planning to be staying for the bulk of the time there (and name of my friend that was already living there on work visa). I don’t remember ever being stopped to check my bags or answer addition questions. Though I might have just been lucky to have not been picked for additional checks.