KPop Demon Hunters theory on the Honmoon & Rumi's powers:
Okay so I am like 75% sure that Celine was full of shit when she said that the Golden Honmoon would remove Rumi's patterns. Rumi seems to be a largely unprecedented hybrid so it's far more likely that Celine herself doesn't know what will happen to her when the Honmoon is fully sealed, especially given that the Honmoon has also never been fully sealed either. It's unclear what all the ramifications of that would be and those ramifications just being purely positive and happening to have the ideal outcome according Celine seems implausible.
To that end, I think the real reason that Rumi's patterns kept increasing as the Honmoon got closer to completion was because something else was going on. The patterns seem to connect to Gwi Ma and the Honmoon seals away everything connected to Gwi Ma, so the most plausible supposition based on that information is, I think, that anything with that Gwi Man connection would get permanently sealed away with him by the Golden Honmoon.
But are all patterns indeed connected to Gwi Ma?
Rumi doesn't hear Gwi Ma the way that other demons do. This could be because she's spent her entire life on the other side of the barrier, but as we see with Jinu, Gwi Ma can grab demons connected to him and yoink them across whenever he feels like it. It could then instead be that Rumi was protected by Gwi Ma's ignorance of her existence -- she didn't hear him because he never even thought to look for her -- but if that was the situation, then Gwi Ma definitely would have grabbed her as soon as Jinu reported on her patterns.
Even after Rumi goes full demon, she doesn't seem to be connected to Gwi Ma or susceptible to his influence.
So my theory is, Rumi's dad was a demon, yes, but not all demons are connected to Gwi Ma. Gwi Ma is just one demon king, the one that the hunters focus on especially because of his habit of devouring souls (and because their powers seem to work especially well on him). The patterns are demonic but only those who have made bargains with Gwi Ma have their demonic natures bound into servitude to him. Thus, Rumi is not connected to Gwi Ma.
But Rumi is connected to the Honmoon. For the past five years she's helped strengthen it. Her voice is part of it and her soul is part of it. When her demon powers start to manifest, they ripple through the Honmoon.
So I think Rumi's patterns were starting to spread because her powers were increasing, because she is a demon connected to a massive spiritual network that is powered by human souls. Her voice failing at the same time was a consequence of her anxiety and burnout, not the patterns actually doing anything to her singing (except indirectly). In fact the reason her demon powers were getting stronger was because the Honmoon was getting stronger.
A fun idea to go with this interpretation: if Rumi's voice hadn't given out and Huntrix had rested up and resolved some stuff before properly completing the Golden Honmoon, Rumi would have still ended up going Full Demon Mode because she would have unwittingly created like a perfect base of power for herself. She would have become a demon king, comparable in might to Gwi Ma. Maybe that's even part of what was going on -- through her connection to the Honmoon, that was sealing Gwi Ma away, Rumi was also unwittingly taking his power for herself. This gets way more fanfic-y but I think it's quite fun to contemplate. Rumi thinks she's going to turn herself full human and instead she ends up usurping a millennia-old demon king and stealing his seat of power. Surprise!
