There’s a lot of businesses that focus on legal compliance. Maybe they aren’t “uber for compliance”, but there’s plenty of compliance centric work out there

There have been businesses that provide compliance services for forever. I would expect ADA compliance to be dramatically simpler and easier than HIPPA or SOC or GDPR.

Like it or not, these businesses will pop up. There are already companies doing similar things for privacy compliance.

What you're describing already exists for most businesses. Try opening a restaurant without hiring a specialist firm to do compliance reviews (rhetorical: is food safety more important than privacy protections?). Bricks & mortar businesses often have a parade of compliance and certification hoops to keep spinning that they live or die by.

The future of the Internet is the present of all other industry. Take all the regulations and government interference that already exists in old industries and apply it to every business online, every commercial website. Nothing can stop this process, it's well underway now and will get radically worse over the coming 10-15 years. The regulation monster has its claws out and is beginning to sink them in (half the room will cheer that today, then complain ten years from now at the burden; oh but they went too far, you see; human nature in action).

Whatever it is you want to build, do it now before the regulators and competition (through regulatory capture) conspire even further to make it far more expensive and burdensome. Hurry, the old days of a low regulation, free Internet are rapidly fading.

A start-up is nothing more than a new business. Has no relationship with market segment or particular technology or whether that tech is new or old.