Hey after 5 years i finally decided to give flarum a try i dont know why its still in beta because 5 years seems to be enough time to get somehow stable even when many modules are community driven. But i am not here to blame i am here to step in so please let me know if y importer Script (Users / Topics / Posts) would be helpfull for you ? its at the moment at a beta phase but i used it to import my phpbb 3.3 with success i added also a slug function to my SQL Scripts and also some specials because for example i do not import private messages and i do not set users to moderators yet because i didn't understand how they are saved.
Maybe there is an dummy for an Flarum plugin i could asume the coding style and make it public at github
Again i am not here to blame it am here to help others to get a flarum running because i got the feeling phpbb is quite a non actual system at the moment.
greetings from vienna