I’m a Pakistani Shia and there is huge amount of Shia there who say things like "May Imam Ali keep you safe, may Imam Mehdi give you health, may Imam Hussain give you blessings" and on. They do not use Allah’s name at all.

Some even go as far as to believe the Imams (as) have the power to create and sustain which they did in the past and still do.

Is this practice shirk?

If they believe that the Imams can do that without the permission of God Almighty, then it is undoubtedly shirk.