I feel the exact opposite. He went from really unique and exciting to almost banal staring with Permanent Waves but really accelerating around the time Hold Your Fire was released.
I can't think of a single singer who got better later on.
I'm not really surprised you feel that way, there are quite a few fans of RUSH who loved their early sound, and then there are plenty of us who feel the band hit it's mark with Moving Pictures. Quite a few people I knew who didn't like RUSH before MP came out changed their mind.
I just think Geddy matured and learned how to get the most out of his voice at that point.