Hello!! I'm assuming you're the one who also sent another ask.. sorry I didn't reply to your DM (I didn't get any notifications at all 😅) but thank you for liking sunsea!!

Books like ☀️🌊.. I suppose mo dao zu shi has that similar trope of self sacrifice and it's also sacrificing something like a soul. Idk its English name do you'll have to find that one out 😅 it's a pretty good story though ^^
Bmjn fics wise.. I'd recommend my dear friend blipblipblup's "furugaye". It's quite similar to sunsea in that it is also a story of souls and self sacrifice, writing so beautifully I was in awe the whole way through. I'll also recommend writingtrestle's "To Sun And Sparrow Song". The plot is pretty different from sunsea and it's so immersive and so so amazing I find myself thinking of it very often.

Sorry I responded so late 😅 thank you for your ask!