as an ultimate class being danmachi seems unneeded, high class are suppose to be city destroyers and ultimate jumps up to continent with super devils being even beyond that so other than sight seeing its kinda pointless to join a Familia
Hephaestus would be a better option considering you can learn how to make a magic swords. would getting a falna seal your potential in anyway? like not being able to get stronger unless your falna is updated that would suck.