Doing this as a browser extension is one thing, but selling an interface to Instagram and YouTube sounds like it's very risky. What's your basis for thinking this will work long term? I see you're selling yearly or lifetime subscriptions, suggesting you think the product can exist. There have been many attempts at this in the past that have been taken down, why is Dull different?
Why wouldn’t making a paid web browser be legal?
Also every single one of these that I've seen before has fallen down in the same way. Chat apps that embed Facebook, third party YouTube viewer for Apple's VR headset, various other third party Instagram apps, etc.