Thank you in advance! 

I've already watched 'Cubic', 'Bromance', and 'Once Upon a Story... in my heart'

When I saw the link I was going to recommend these but u already saw them , right now I am searching for the same genre so glad that I saw u r post 
^ Aww! I'm really hoping someone drops in some recommendations as well!
I think you've hit the main ones but some with the theme of current/former gangsters:

The Masked Lover
Love Around

You won't see much action/violence in these ones but it is clearly established that they are gangsters or slightly reformed gangsters and there is some action scenes.

Lovers - http://mydramalist.com/1069-lovers-2006
Cruel Romance - http://mydramalist.com/8963-cruel-romance
Lady & Liar (probably a happy ending but I'm not sure since I haven't finished it yet) - http://mydramalist.com/8898-lady-liar
When A Man's In Love (I can't remember the ending though, I'm pretty sure it was a happy ending?) - http://mydramalist.com/6063-when-a-mans-in-love
Morrasoom Sawat - http://mydramalist.com/16169-morrasoom-sawat
U-Prince The Series: The Ambitious Boss (it should have a happy ending, but I haven't see it yet) - http://mydramalist.com/16469-u-prince-the-series-t...