Satoshi gifted us a *theoretically perfectly anonymous* transaction system.
sats do not flow "from the second input to the third output".
Rather, sats on the input side get destroyed, and sats on the output side created. It's not a flow of sats, but more like a wormhole.
We have been stupid and made bad use of this miraculous technology. We have always made small single user transactions, and if there is a low count of inputs and outputs, the k-anonymity of this transaction is also low.
The extremely simple solution to maximize the privacy guarantees of Satoshi's transaction system, is to increase the count of inputs and outputs in a transaction.
That's it.
That's the trivial solution to Bitcoin's privacy "problem".
BIG TRANSACTIONS.