Certainly his minor characters are very shallow (there are like 1200 of them to your point).

But his main cast have all been developed quite thoroughly as the series has progressed.

Rand, Egwene, Mat, Nynaeve, Elayne have all changed pretty significantly over the course of the books and to a lesser extent Min, Aviendha and Perrin.

Mat probably the most dramatically, but Rand, Egwene and Nynaeve are all very different people from their EF days! In Mat's case he's like a totally different person! Even before he gains the 'Finn memories he's already shown the signs of being the heroic person he later fully embodies.

Certainly RJ kept to specific personality archetypes, as evidenced by the various similarities in women of the same Ajah or people of the same nationality (i.e. all Cairheinin are schemers), but he did have a growth trajectory for at least the main cast. Personally I've never thought this was a weakness in the writing, I find it quite thematically appropriate.

Note how people start changing in small ways once they are forced to communicate with outsiders? Red Ajah women like Silviana and Pevara start cropping up, Dobraine and Caraline show us that Cairheinin aren't all self-serving schemers etc. RJ did introduce change, just in a subtle way, using specific characters to change the archetype of a larger group.