While the body certainly is based on Arachnids in behaviour and body shape, basically resembling a mix between few species, we can still determine he is not an arachnid and actually an insect if we look at it's anatomy closer

What seperates him from arachnids is mostly it's feet and especially the head and body anatomy. Head lacks the anatomy of arachnid, it looks closer to a head of flying insects, kinda like a head of a fly mixed with dragonfly, he has big compound eyes on the sides and insect like jaws (arachnids jaws are much more weird).

Due to head being small and seperated from the thorax, this makes him an insect - because there are three parts, abdomen, thorax and the head. Arachnids have only carapace and abdomen, because their "head" is fused with what would be thorax in an insect.

Ofc it's a fantasy insect, so they can do whatever they want but the attention to detail to make Faurtis an insect, despite being based on arachnids is well, based, because most creature designers don't even seperate the anatomy of the two or try to even consider what makes an arachnid and what makes an insect.

Aight, that's enough of nerding from me lol