I'd like to ask why it takes years to build a single large grid transformer. Is a question like that on-topic? If it's not on-topic here, would it be on-topic on a different stack?
I would think so; this stack is "Electrical Engineering", not "Electronics Engineering."
Like all aspects of electrical/electronics, large-scale-grid is its own separate "local cluster" in the "electricity galaxy." I've seen a few such questions answered. But the quality of the answers totally depends on which experts (if any) happen to see the question. The more obscure and field-specific the question, the less chance anybody else will know about it.
That said, folks here are quite intelligent. Very rarely is this stack totally clueless about anything ranging from "vacuum nanotubes" to beer...
I agree with others that it is on-topic here, though I suspect better answers might come from engineering.stackexchange.com since the issues involved are not so much about the electrical properties of substation transformers but about the process of specification, procurement, and manufacturing of large-scale unique-design equipment.
FWIW, I would also be interested in seeing more questions and answers within this realm of EE on this stack. Although electronics is a hobby since adolescence, my profession is EE within environmental public infrastructure (i.e. water, wastewater, solid waste).