I think you are not counting: QA, user experience, DevOps, technical writing, security auditing, user training, integration support...

What happens if the system collecting all of your data has vulnerability? suddenly all of your apps and data are owned.

What if it's inefficient? then you have DDoSed yourself.

Non-functional requirements cannot be solved with a functional requirement mindset.

What if you are losing data? what if you aggregate it incorrectly? what if data is ingested in the wrong order? ... you get the idea.

It's not a 3 people problem. When you take on a hard problem like this you have 10 different things to cover and you need to pick all of them.