>Why would complex financial agreements keep the Romans from large scale automation?
Because the ability to amortize your costs is a boon for outpacing smaller firms. Basically, the more cash you can get your hands on that you can defer the lump sum payment on the more you can build out and thus the more you can produce. It basically becomes a positive feedback loop (this includes state subsidies indirect and direct which I'll leave as a generalized foundation for the sake of a clearer argument).
>The Romans had modularized home construction so that they could scale and that doesn't happen without financial incentives and some level of an industrial revolution.
Modularity was born out of the immediate demand for the product (housing). Note that modern, capitalist, economies build on the basis of volume whether it's housing, smart phones, clothes, and so on. And it can do this due to the fact that costs are amortized over the payment of debt along with the state subsidization as mentioned. Essentially, capitalism fuels itself through debt and state based subsidies (ex. interstate highways subsidize trucking yielding higher profits than would be possible if interstate highways were wholly private). This includes the inducement of markets (ex. prior to the trans-continental railroad the US markets were regional at best and most international trade was by sea for commodities such as cotton, gold, or ores).
Also, Roman upper class had no social need to turn their profits into more profits. They would often build themselves villas, have lavish feasts, and many other temporary luxuries in their place as their social standing was more based in that than in sheer monetary/accounting wealth.
>Rome's problem was always crony capitalism and the fact that any Voltaire's that might have existed would have been violently executed by the state. Without freedom of thought you have no Industrial Revolution.
This here is your primary error, capitalism did not exist prior to the the 17th century (merchant capitalism) at the earliest. Yes, there were loans but nothing to the complexity or legal arrangements that even a modern small business loan has in terms of legal and social dimensions. Today, debts can be carried by corporate entities. In the past though, loans were only to be held by the person or people who agreed to them. It was a rare concept that loans or debts could be owned by someone else (ex. one nation conquering another taking on their debts which is a new concept) which is an important construct for financial capitalism to emerge from industrial capitalism.