I come to a similar conclusion but the opposite way, almost. I tend to design scenes specifically for the RS that, when I come to writing them, wind up incorporating at least one other throughline. I’m about to write an RS scene now (that’s all about whether a work-centric relationship can become a friendship) and I already know it’s going to feature quite a bit of the IC throughline (and a little set-up for the OS).
The RS is the most difficult of all for me, so I find it helpful if I have a plan. Even if it’s just as basic as ‘Can this relationship become something more than a team-mate bond?’ or ‘How does the relationship hold up when both get the idea they’re attracted to the other?’ By the time I get to writing that scene or sequence, I can see what’s happening in the other throughlines and just discover the scene as I go. Since it’s the emotional core of the story, it tends to feel more ‘real’ to me if I just follow my subconscious to write the scenes.