You're almost correct!
Spoiler! (Basically what I can rmb, some stuff I might misremember but this is the gist of it)
It was actually in her first life that Heesung was an idol with Yeonwoo. Idr what happened to Heesung but it cause yeonwoo to get a system and go back in time but it messed up and made Heesung older than her, which is the life where she's a fangirl. Because of her devotion to Yeonwoo, heesung gained a system and reverted back into her actual age in the 3rd timeline which we are at now however 2nd life Yeonwoo's system tormented her so much which is why every now and then we see Yeonwoo's personality change to the one from that time
Another addition to the spoilers,
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Basically, an accident happens where Heesung saves Yeonwoo at the cost of getting a leg injury, so Yeonwoo goes back in time so that she can avoid the damage. However, Heesung's injury becomes worse because she didn't move this time. This causes Yeonwoo to regress more times and making things worse each time. In the final repeat, she asks system to erase her memories of the regression and to give Heesung a normal life.
From there, we go to chapter 1 where Heesung was an office worker and a fan of Yeonwoo. During the survival show, a glitch happens when Yeonwoo looks at Heesung, which causes her to remember her past lives and eventually allowing Heesung to regress to this current life.

This is Heesung’s second time debuting. In her first life she just lived as a simple fangirl, went back in time, and tried debuting with Yeonwoo. I think she fell ill or died due to the penalties of the system and it must’ve been because of Yeonwoo. Yeonwoo finds out about the system and is now momentarily going back in time in her past life’s body and trying to stop Heesung from debuting and sacrificing her life for Yeonwoo. Heesung gives up her life and future for Yeonwoo’s dream and Yeonwoo gives up her dream for Heesung’s life. This is anything but casual these two are in love.