That is a very confusing set of criteria. You want someone like two specific artists in the title, then name two specific bands/artists in the text that are radically different from those in the title (not to mention each other tbh)

That being said, you might like Norah Jones. Haim has a similar vibe to Merton, though I'm not sure either is specifically "romantic" overall.

There's Sade for another smooth soul voice.

Toni Braxton might scratch an Adele-ish itch. At least her early stuff.

Kinda spitballing here. But there wouldn't be an Adele without Aretha, Nina Simone, and Etta James. I would even argue that those three are the greatest voices in soul, and rank high with any genre open to comparison. Shit, even if you don't like them, you gotta listen to some of their stuff (assuming you haven't) just to understand the foundation of soul. Yeah, there are men that could have their own top three like that, but op specified female singers by their examples. (Shout out to Barry White and Marvin Gaye in particular)

Ooh! Joss Stone! Gotta give her a listen. Not as smooth as some of the queens listed above, but just as much depth.

Corinne Bailey Rae is worth it too.

A bit less well known, but Allison Young does a solid retro-jazz style that's not quite in the same zone, but her voice is so good you should try her out anyway.

I dunno, my brain has hit a wall trying to circle back into Motown, which is n amazing musical destination to hit, but not quite the same vibe as Adele brings, but informed the underpinnings of a lot of later singers. Diana Ross alone could take the crown in that regard. But Motown covers a wide range of styles, which makes it a great music era/genre/whatever-you-wanna-call-it to dive into when looking for romantic music.