Personally, putting the older brother aside, I don't think the younger brother has a compatible personality with Wooseo who is a lot more mature. For someone who loves so deeply and carefully like Wooseo, Jigeon who is similar to him can fully recriprocate the intensity of his feelings. Anyways, who knows, you might get some side stories of Jiseok?
I think both brothers have a maturity problem, but Jiseok was at least mature enough to back off and step aside to protect Wooseo from any more pain. Yes, Wooseo loves deeply, but Jigeon’s “love” is more like an obsession, and he doesn’t care who he hurts as long as he gets what he wants. I don’t feel like they’re compatible at all, tbh.
Yes and no. Jiseok didn't necessarily just back off to protect Wooseo from more pain. He's now realizing that Wooseo likes Jigeon more than he ever liked him, and ultimately conceded. Jigeon is not the less mature for decisively taking a last ditch gamble for what he truly wants. Maturity is when you make a decision and stick through with it and reap the consequences of what you sow. The reason Jigeon let both of them realize is because he wants Wooseo to know about his true personality. Even though he is the chessmaster that set up the board, he didn't force any of the pieces to move. From the very beginning every character made their OWN decisions. All these decisions were STILL Wooseo's decision. Would this really have been a sweeter story with Jiseok? Arguably the time Wooseo spent agonizing over Jiseok is just as toxic for him, other than the fact that Jiseok only started wanting Wooseo because he realize Jigeon was taking him away. Jigeon may be a means for an end kind of guy but he's not bad for Wooseo. In the long run, after they make it through this last hurdle, they're definitely going to have a long and good relationship.
childhood friends to lovers is much sweeter than falling in love with your best friends toxic and manipulative older brother bc a magic ring turned you into a forced proximity trope. jigeon is not even a little mature, he's disgusting. you can say the characters were technically making their own choices but you do understand what manipulation is right? he is straight up a bad person. sadly i'm not wooseo or the author so i'm unable to affect the terrible decision he's making, but i can still dream.
You don't have to like him. This has always been a villain's perspective story if anything. I can understand preferring a sweet and wholesome BFF story, but I think most of us came here for the dark plot, and it has never shied away from being presented as anything but. Even if Jigeon was manipulating the scenes, he doesn't control people. Aside from fight or flight, when people are put into situations, their personality comes through and their choices are their own. Even without Jigeon, that's still their decision in that circumstance. There are people who are doing moderately, and aren't willing to help others, and there are people who are absolutely broke and will still give you what little they have. Did anyone force Jiseok to ditch Wooseo for Mina? No.

I wish the younger brother was endgame. The older brother sucks, and I always prefer the childhood friends to lovers trope. Blah. I wish the younger brother would at least find his fated partner before the end of the manhwa, but I don’t see that happening.