Sounds really cool!
What if you did something where you have a big list of features including: materials, sizes, and complexities, and each had a score (e.g +1, -2, +3, etc.).
And then there could be two ways to build. People choose the intended Tier, and choose a combination of features that that add up to that value. Or they just choose the features they want and that determines the Tier.
I think you might find it difficult to set established Tier values for some of those things as you are right now, with small being 1 for example. Because a small super nimble golem made of indestructible metal is going to be higher Tier, even if it is small.
Just a thought! I'm excited to see what you come up with.