this might be good for you , its not really fully yakuza but it has some mafia stuff http://www.mangago.zone/read-manga/walk_on_water/
and this manga too : http://www.mangago.zone/read-manga/saezuru_tori_wa_habatakanai/ its exactly what you're looking for excepted for the happy ending , idk if it has a happy ending because its not finished
http://www.mangago.zone/read-manga/raise_wa_tanin_ga_ii/ I mean you might know this, but if not you're welcome (▰˘◡˘▰)(▰˘◡˘▰)
http://www.mangago.zone/read-manga/mangaka_to_yakuza/
this one was cute, bxg manga, but it didn't have that much drama
well, in the saezuru , they're both complex adults and they're in a yakuza context , wich is a very violent and dangerous environment. So there are not much couples in these context because when you're a yakuza you don't have time for really getting together with one person , and you don't have have a proper mindset to install a normal relationship. The characters have horrible backgrounds so their way of thinking isn't really healthy, and they're not really a couple; but you asked for drama and tragedy that's why I recommended this.
recommend me a bxg manga/hua/hua where boy is yakuza