Do you launch on all platforms in one go? Or one by one?

Don't think it matters unless you are not sure about how well your server can scale πŸ˜‚

It depends. If I am skeptic about the product/idea/implementation, I tend to launch it on one platform first, just to check what the general public feedback about it, is.
When that is done, I launch it on other platforms simultaneously.
This is sort of a safe approach, it saves you from a complete launch fail, but also, on the other hand, prevents your product from reaching its full virality potential.
I did it with #vscodenotebook, I was unsure about it so I only launched it only on Reddit. It went quite well and I lost the chance of going more viral than it actually went. πŸ˜‚