Glad you like! It remains one of those series that defined my childhood.
I agree his characters start as two dimensional tropes. That being said, as the novels progress he does a fairly good job (for a fantasy writer) in bringing forth their backstories, catharsis, and evolution. Belgarath, for instance, is definitely not Gandalf the more you get to know him. Polgara's relationship with Durnik (and Durnik's relationship with Garion) are also exceptionally well rendered. In fact, it's how Eddings' describes Garion's evolution from boy to man, and how the morally ambiguous characters around him encourage that evolutionary catharsis that really gives the series more depth than most fantasy fare.
It's notable that in Eddings' later series, this dynamic was completely missing, and his books suffered greatly for it in my opinion.
As to world building, what impresses me about Eddings is his detail in describing the cultures he creates, and his capacity to make them come alive when seen through the eyes of a farmboy. Unlike Tolkien, the cultures are not defined as black or white, but in varying shades of grey, and unlike a lot of the other fantasy out at the time, it wasn't just a rehash of LOTR. While I fully agree that what "world building" has become in current times vastly overshadows Eddings' work in terms of scope, at the time, his use of low fantasy to render his world gave it remarkable vibrancy and a life that I didn't really find in other writers of the time. For instance, I can tell you they rode dragons in Pern, but that's about it, whereas I can literally hear the god Mara sobbing in the empty ruins for the slaughtered Maragor from when I first read the Belgariad as a tween.