Even if "all the data" were accessible by all citizens, there would still be a massive asymmetry present in the government's ability to process that data into useful information.
Realistically, people would build tooling — either free, ad supported, or SaaS-based — that lets the average person synthesize publicly available data. Just making the data available allows motivated individuals / groups to do a lot of the work upfront, and then share the fruits of that work with everyone else, either for free or for a fee.
You see this already today with data from census bureau, FRED databases, COVID tracking tools, think tanks, etc.
Government and corporation. The FAANG companies clearly have enough processing power to churn an entire nation's big data streams.
Although, if one's goal were a town or even an individual city, I bet enough infrastructure to sift the data is buildable or rentable by individuals or small groups of individuals.
It becomes an interesting world when non-profits can afford big data resources.
I think that's our best option for avoiding oppression. Case in point, video of George Floyd. If we had widespread public (by the public) surveillance, I think people would feel safer. If someone is "disappeared," hopefully there'd be evidence that the ACLU could pursue.
The "extraordinary rendition" program the CIA was using to disappear terror suspects into places they could be tortured was discovered in part by airplane-spotting hobbyists. Because planes are extremely hard to hide, and there are people who watch airports to see what takes off and lands for fun.
When they pooled their data, they were the first group to notice the military had started running flights to and from locations they didn't normally fly, and it didn't take much investigative journalism after that to discover those planes were carrying people.
(Not sarcasm)