I'm pretty sure this is an unpopular opinion, but the "lack" of content in Lucanis's romance makes complete sense to me. Honestly I never saw it as lacking at all. I can understand why some people do and of course it would've been great to have more content, but to me, there is narrative sense to the way his romance is written.
Just looking at the last year or so of his life, Lucanis has been imprisoned, tortured, and to top it all off, possessed. He is sharing his body with a demon and constantly fighting to make sure he keeps control. And when he finally comes back home? Immediately finds out his grandmother is supposedly dead, and for all intents & purposes, it's in retaliation for HIS jailbreak. Already he is picking up broken pieces of his old life and cutting himself on them. It's no state to be jumping into a relationship from.
I've said before - and there's plenty of dialogue from him supporting it - that Lucanis spends most of the game trying to pretend he can go back to his old life; that he can be the person he used to be.
"I thought I still had this."
"Whatever else I am, I'm supposed to be a professional."
"I don't fail my contracts."
All of these things add and add to his turmoil until it's set to topple right at the start of the "Inner Demons" quest. He finds out his grandmother is alive! He should be happy! Instead he's caught in emotional upheaval, overwhelmed by everything he's been pushing off, and he has to make a decision, he has to keep going, he has to has to has to. And he can't. To the point that Spite has to intervene and say the one thing Lucanis won't.
"Help us."
And then, after getting to Lucanis, Rook literally tells us the man's struggle outright.
"As bad as the Ossuary was for you, it was better than the alternative. [...] You could solve those problems with a blade, but healing again? [...] There's no simple answer there. And if you fail, you could hurt the ones you love."
Lucanis was raised in violence. He still has a good heart in many ways, but that doesn't change the fact that he was more than likely never taught the emotional skills needed to navigate his problems any other way than with a knife. Which means that for 90% of the game, he is not ready to open his heart to someone. Even the 'almost kiss' scene is Lucanis attempting to replicate some of who he was, until something in him realizes that kissing Rook wouldn't be a one and done situation. No no, he wants them. And the minute that hits, he pulls away again, until the time comes in the endgame that he realizes losing Rook forever is scarier than never making the attempt to be together.
TLDR; Lucanis is a traumatized slow burn with one hell of a payoff in the end, you just gotta be patient with him.
P.S. - Once again, this is just my opinion, you are free to agree or disagree as you'd like; this is just based on my own experience with his romance.