Which room should I lock?

I'm guessing the answer is the smith two nodes away from Architect because that's where the loop ends, but you'll probably get better advice from someone else on here. I never fully comprehended the temple mechanic.

Maybe im wrong but lock the smithy and golem let the rest destabilize and continue?

I am far from a temple expert but it looks like you looped. I would expect you have to lock the node in the loop that is directly connected to the tail of your snake, so the smithy.