Does full automation still provide a better ROI at small scales?
There is definite value in automating manual tasks and placing systems under configuration management that is done using code, not using paper and human intervention.

One huge benefit is the reduction in the amount of rework. You can consider any problem caused by a human mistake in repeating the same sequence a case of rework. As well as any bug that needs to be fixed more than once.

Rework is costly, and there has been some research done to quantify its cost. You can read about it in this whitepaper - https://devops-research.com/roi/

The same whitepaper also quantifies the cost of failure for services/systems, and the slowness to resolve failure. All of which are greatly reduced by having a robust automated process that prevents human mistakes from ending up in production systems.

The topic of automating manual tasks has been also investigated in the State of DevOps Report that has been issued every year since 2013. Most of these are available at https://devops-research.com/research.html as well (with the exception of 2013). In every report it was mentioned as the biggest driver for excellence in high-performing organizations of all sizes. There are plenty of reasons and explanations that can be used as proof for your case.