tree trunks become strong when they are properly staked. If you had it tied to the nursery pole it came with, it didn't get strong. The poles that come with potted trees are intended for transport, not for long term use. No one ever tells you that, though. The diagram in the link shows how to properly stake a tree so that it becomes strong. You may need four ties instead of two.

Here is a link that might be useful: how to stake a tree

Why is it leaning? Is there shade that makes it reach for the sun? Just curious. I started one from a seed 2 years ago. It's now in ground down the hill, and it has no stake. It is in full sun, and it grows tall with new growth everywhere. I don't expect avocados from it for many years, but it does NOT seem to need a stake.

Every afternoon (this is wine country) winds kick up. It only grows stronger against the wind.