Both Daeron and Celegorm are put before the audience (and Lúthien) as potential alternatives to Beren. Naturally, Lúthien chooses Beren. But while the audience may think that Daeron and Lúthien are a better match at first...they aren't going to have any of those qualms about Celegorm - he is clearly bad news. So, not only is Daeron part of a potential love triangle, we're going to introduce another triangle immediately after. So, I think we should really emphasize the long history of friendship between Daeron and Lúthien - he knows her, and so the worries and concerns he has about her situation come from that place. That will contrast strongly with Celegorm's viewpoint, as a stranger who just met her, thinks she's beautiful, and hopes she'll come around to his point of view.
Think of it this way. If someone announces a sudden engagement to a scruffy guy she just met on Facebook, their friends will congratulate her, tell her the ring is beautiful, and wish them well. Because...casual acquaintances, co-workers, former classmates...they don't have to live with the consequences, and will just trust that she'll figure it out - if she's making a mistake, she'll learn the hard way. Very few friends will pull her aside and say, ummm...what are you doing?! The thought there is that people tend to mind their own business, or express their concerns to their own friends. They'll talk to *other* people about her crazy plans, but not confront her over it. The only people who would directly question the girl would be very close friends or family, who are concerned, because they don't want to see her hurt, and are willing to risk her being upset with them in one conversation if that is enough to avoid a bad situation that they see.
Most abusive relationships have red flags, but most abusers are careful to keep these red flags hidden from close friends and family for a long time. Being charming is necessary to mask the ugly behavior. There will always be a lot of excuses and explanations as to why things are happening the way they are. And any disagreement will be used for future isolation. There is enough tension in those situations that it is difficult to confront anyone involved, because the likely outcome is being cutoff.
So, strangely enough, Daeron's concerns about Beren - hey, you two just met, he's a scruffy mortal, he stumbled into our land from the war-torn disaster he calls home, I don't trust him....these sound like reasonable things to be concerned about, and the type of thing that he, as a good friend, could discuss with Lúthien. Becuase Beren is *not* abusive, nor seeking to manipulate Lúthien, he is not going to prevent Lúthien from having a heart-to-heart with her good buddy Daeron. And because she values his counsel, she's not going to simply get mad at him for questioning her new-found love. She'll probably tell him he doesn't understand, but also thank him for being a good friend and sharing his concerns with her.
So, I think we start with 'concerned friend' when it comes to the starting point of Daeron's story. Of course he loves Lúthien - everyone does, and she is very good friends with him. But perhaps the idea that it's jealousy, or 'Lúthien should choose me' is not really the primary angle, here. It's probably not absent entirely, but I think he could object to this relationship on other grounds, and be genuinely worried about Lúthien's wellbeing.