No, you’re right. Their plot is what the f/f couple would usually get, it’s pretty separate from everything else. If you took Renata out of the show, nothing would really change. It’s a cute romance for Guille and a way to cause friction between him and Eva but she’s not doing anything plot-wise.
Juliantina is one of the most central aspects of the show, you need them as an actual plot device to reveal the body-switching and the connection between the cartels.
@dealanexmachina pointed out to me that Guille/Renata might actually be another way to appeal to the audiences for Juliantina, to parallel the rich/poor trope viewers are already used to seeing and to have them to compare directly. If you’re going to root for the m/f couple then you should root for the f/f couple too. Juliantina are paralleled both by Guille/Renata as well as Lucia/Jacobo (Leon), where Lucia and Juls are from the wrong side of the tracks and brings complications to the relationship while Leon/Val are still besotted and willing to do anything for them.