#also i sort of think his final wish isn't to die (if he sacrificed hinself for someone it'd be a different matter tho) since in vol16 he

[noa-ciharu]

I think you were right about self-destructiveness @theanimepsychologist especially since:

The moment Kamui began worrying about Subaru's health Subaru deflected it by changing a topic to Kamui's studies. He's uncomfortable by people caring about him (like his grandmother) since he himself doesn't care what happens to him. Also subconsciously probably doesn't want people getting emotionally attached to him since he has a death wish. Yet still he and Kamui did form some sort of emotional bond but still Subaru couldn't accept in his mind that he was cared for.

He stops smoking briefly (while Kamui is present) but starts almost immediately after he leaves. Subaru is still kind, he doesn't want Kamui to worry about him. Since he recalls "it's bad for your body" he's aware he's engaging in self-destructive behaviour and in that moment maybe sort of wishes he wasn't "sick in heart" like that. That he could heal somehow. But of course that's just a temporary thought since Subaru goes back to smoking immediately and looks up wistfully to the trees (sakura trees?). One moment like that can't possibly undo years of isolation, guilt and psychological damage.

So I think Subaru sort of lied in that moment because he didn't want to upset Kamui (he's still kind and cares about the boy). It's intresting how Kamui picked different brand part but didnt such obvious lie. Also to some people nicotine acts like a sedative, sort of calms them down. So it's also possible that, beside seeking self-punishment, it has numbing effects on Subaru, sort of calm his emotional tension and guilt down and by extension makes him more focused and concentrated (and by extension "stronger", yet if this was what he meant then it's some serious mental gymnastic on Subaru's part).

About the random brands, it sort of implies Subaru doesn't care what he uses to 'harm' himself with, nor has much care for stuff around him at all. Of course, I saw theories that he picked up smoking as a subtle reminder of Seishirou (not all memories are mental after all), but smoking exclusively same brand as he did would stir up all painful memories. So random ones are more safe (?).

About Kamui latching onto Sakurazukamori topic, I'm not sure how he made that connotation but he most likely concluded: Subaru smoking to be stronger -> wanting to be seen as valuable and powerful (and something other than object) by Sakurazukamori at any possible cost (uncaring for what happens to himself). It'd be fitting since Kamui wants to bring "Fuuma back" uncaring for what happens to himself in process. He'd want to get (mentally) stronger for that. That's one of Kamui's line of thoughts. That's why Kamui could understand Subaru's obsession to some degree, but at same time thought Subaru wanted some kind of recognition, (emotional) closure or answers from Seishirou rather than to die at his hand. It'd be out of Kamui's line of knowledge since he couldn't relate to that part, unlike the obsession part which was familiar to him. That's probably why he didn't see through that lie.