Is this how to solve the scalability of Bitcoin?
Stacks seems to be more about adding smart contract functionality rather than increasing scalability. But yes, generally speaking I'd reckon Layer 2 solutions, though not Stacks specifically, would be the way to go.
It is not a hard fork so it does not need network consensus.
To clarify for OP: The hard fork is happening on the Stacks Layer/Sidechain, not Bitcoin itself. So the Stacks team can do all the changes they want, without affecting the Bitcoin base layer.