Ubigi and airalo are both amongst the cheapest (~100-200yen/GB, depends time & how much data of course, if you only need 1GB for 30 days it's different story). IIJMio can be cheaper but hoops to jump thru (but it's a native network)

Myself (and a few users) had issues with airalo Japan. I had no issue with ubigi (ubigi provides geo located Japan IP but it still routes via Singapore with some latency)



Hyoerdia is no longer useful (they no longer offer timetable results, only generic avg times). I use Japantime&googlemap

https://www.japan-guide.com/news/0057.html