You forgot the step where you even need to identify that these things already exist, and whats in them.
Are there governance requirements around the data in them? I'm not familiar with US law, but we're talking about PII in this example so what legal and privacy hoops need jumping to copy the data?
Thats before we get to "Can an person with this specific learning disability, or a person who doesn't have a fixed address navigate through this form?"
As much as this contract will certainly be a barrel full of pork, and Deloitte can go eat a bag of hammers, I feel this take is a little naive.
A commercial service is optimised for profit, not broadest accessibility. For example Eventbrite won't spend money developing special features for very specific niche user cohorts because of diminishing returns.
Government websites can't work that way, they have to be accessible and usable by all citizens. Think of people with different learning abilities or language proficiency, the likely environments the users might be in, and their general technical abilities.
Here in the UK there are strict rules about use of clear language, well thought out user flows, how to ask for certain types of information etc that has all been backed by years of research. We have a whole government department that covers this and will audit your new service to make sure it is good enough.
Having done this as a web developer for our Government, it was a massive eye opener to have a copywriter pick through the language I used in a form flow, and point out how confusing it could be to some groups of people. Or indicate that the method by which I was asking for an address made no sense for certain remote locations, and it needed to.
A few specific things to point out on Eventbrite in particular. It seems to me that you are likely have a whole bunch of other requirements too. Off the top of my head you'd need more powerful management tools to control: simply setting up so many locations and dates, who can book when (patient cohorts, priority groups), the logistics of vaccines and vaccinators, tracking vaccine batches used for each person.