@elevown
I actually watched the Killing Slimes anime as I read the LNs for it and, with two exceptions, I think the LN is much more enjoyable. The bit with the Minstrel is better in the anime just because of the music. Then The Witch's Cafe ends up more enjoyable in the anime because they waited until the end to do that part so more characters end up involved in it which was just nice. In the LN not all of the main cast had been introduced yet at that point.
Outside of those two, yeah I highly prefer the LNs. You get a lot more detail in various moments, and a lot of moments happen a little differently. Also a few chapters got skipped outright by the anime. One of my favorite moments with Laika happens differently in the anime and it actually kinda bummed me out a bit cause in the LN it's a funny but sweet moment while in the anime it's enirely played for laughs.
Far as translation goes, the official English is up to 9 or 10, and there's a Beelzebub spinoff that has 1 volume.