The only game I can think of to do this as a part of the main story very well, rather than as an optional “Score relationship points to earn this person as a meaningless prize” is in Trails in the Sky 1+2. Old game by now, but it really stuck with me.

The challenge is to involve character relationships in the story itself, in ways you don’t expect; have that person who fell in love with the main character lie to protect them from a murder charge, and then be arrested for treason; have a villain that takes one love interest hostage, only to realize someone else cared even more about them and is far more dangerous, etc.

Xenogears is my favorite romance story in gaming. Amazing depth to it.

I also liked the Bastila romance plot in Knights of the Old Republic. Some neat Force shenanigans going on there. Tali is my favorite self-insert Bioware romance, though.

The first RDR counts, I think, and did it well.

I’m not sure you’d call it a slow burn but I really enjoyed how the relationship developed in Prince of Persia: Sand of Time. Pity we never got a proper sequel.

Geralt and Yennefer in Witcher 3. Felt like a real slow burn until two-thirds of the way thru the game.

I liked the romance options in Dragon Age, as well as Cyberpunk.

I still think Olimar and Louie are going to do it some day.