Well there is gonna be more to the Spirit Turtle if your as Volume 6 and 7 are dedicated to it. Others may tell you the manga is enough but personally I prefer the LN as it’s got more detail and there are also things that the manga cuts out. Also like NextMore said the LN is exclusively from Naofumis point of view. Anytime you see narration going on it’s him thinking to himself or explains things he sees. There are bonus chapters at the end of some of the Volumes that are from other characters perspectives which is a breath of fresh air at times. Volume 1 is about Raphtalia and her time as a slave and Volume 2 is the Hot Spring arc both of which are form her point of view. There are a couple of scenes from the Hot Spring arc they cut out from the anime as well.

As for the relationship between Raph and Naofumi well it feels like it’s being more pushed into that direction, however Naofumi still sees himself as her surrogate father.