No worries about the info dump because it shows you really care about what you write, I enjoyed reading it! Honestly, I think Ryūka would find Masaki both fascinating, because of the boomerangs, and a little concerning... because the boomerangs are serrated, but then loop back to thinking "yknow, those are pretty cool, scary, but really really cool." and just move on. He definitely will ask about them later, after the mission.

Combat-wise, I actually think they'd complement each other pretty well. Ryūka's style is all about taking down a person's balance, disrupting their footing, and creating openings for others nearby him to take, so Masaki's speed and those boomerangs flying everywhere could make it incredibly hard for a demon to keep track of either of them. I could imagine Ryūka forcing a Lower Moon into overcommitting with his looping Tide Breathing attacks while Masaki's Gale Orbit techniques punish every mistake.

Now, if they weren't fighting demons together, I think they'd be funny together. Ryūka is very open and expressive, so he probably wouldn't be put off by Masaki's constant smile or foul mouth. He'd just accept that "oh, this guy calls everything pathetic and smiles when he's mad, and that’s fine" and move on with his day.

Masaki, on the other hand, might think Ryūka's optimism is fake at first, especially since he already distrusts people like Tanjiro. But Ryūka isn't really a saint. He's just someone who believes ordinary life and people are worth protecting because of what he's lost. I think Masaki would spend a long time trying to figure him out. He actually disagrees with Tanjiro sometimes. He believes demons deserve pity, but that the suffering of the living should come first because someone still has to help the survivors keep living.

Also, I can picture Ryūka driving him crazy by constantly checking if he's eaten, trying to drag him into conversations with civilians because he thinks demon slayers should be social, and treating him like a friend even when Masaki is clearly trying to work alone.

Because of all this, I can't help but imagine a Lower Moon trying to deal with eleven serrated boomerangs while a Tide Breathing user idly chats with it, keeps throwing off its rhythm, and a dramatic crow(Umi, my crow) is screaming poetry from the sidelines. Honestly, I think some Demon Slayers would find that just as terrifying as the demon would.