Straight to the point. I mailed Yostar about the Normal Battle OST (Sardegna Empire) and Daedalian Hymn Event PV soundtrack to get the information such as artist name or credits of the soundtrack, but I'm not happy with their answer.
They replied
"Dear Commander,
Thank you so much for your support and am sorry but we have no such info and we can't provide any game-related resource to others. Thank you for your understanding."
Those two soundtracks are probably the best soundtrack of Azur Lane in my opinion.
I'm not planning to pirate the soundtrack, if I knew the artist, I would support it.
If anyone has the credits/information about those two soundtracks, I would like to know it, thank you.