Oh, yeah, I’m not saying it’s bad, but it’s… kind of what TVTropes calls Early Installment Weirdness; it feels very different to what comes later.

Could you explain? I don't see much similarity between them.

In both protagonist is a super powerful boy, unintentionally bringing the world on the brink of Armageddon?

Well, I guess...? Tonally they feel completely different and Good Omens draws a lot of its DNA from The Omen (and, wonderfully, Richmal Crompton) whereas I don't see any of that in Sourcery.

I find it hard to see Good Omens as a do-over of Sourcery, but I guess they do have something in common.

Probably worth noting that Pratchett wrote most of the "Adam and the gang" sections of Good Omens, while Gaiman was responsible for the riders and the more "mythological" parts. So I do see a pretty clear throughline from Sourcery to the Pratchett parts of Good Omens.