I guess what I'm looking for is the 'optimum setting', which race/class that would better suit this romance or one that will lead to a happy(er) conclusion. I thought about RogueTrevelyan/Cullen pairing, but I'm just dying to play as a Dalish Mage \(^o^)/ finally.
I wouldn't call it "optimum," but the "least risky" match for Cullen would be a non-mage human. There's no culture or religion shock, no mage and duty conflict, just about zero reasons for late-game relationship incidents. Unless the whole romance ends in tragedy anyway. 
But if that's not what you wanna play, don't force yourself. I'm romancing Cullen with a rogue Trevelyan because that's what I want to do, not because it has less potential for conflict. If what you really have in your heart for this romance is a Dalish mage, go for it. Live your character the way you imagined her.
The mistress ending by itself is awesome though. I usually marry Alistair to Anora when I'm doing it. I had a HN mistress mostly because she wanted the freedom that it offered.
Yep, my Warden for this playthrough is a human noble and settled for being Alistair's consort for this same reason. Well it was more like "freedom to do her duty as Warden-Commander," away from the duties of monarchy. She made (hardened, obvs) Alistair marry Anora for the political symbolism the nobility wanted, knowing they hate each other and Anora'll never be a threat, became Chancellor so she and Alistair together have a stronger voice than Anora's, and is quite content keeping her real relationship with him away from the public eye.
#NotAllCouslands want to be queen. 