However Monero has shortcomings, the main one being its auditability.

Some Monero developers have created a new project which tries to solve those issues, and add zero knowledge proof to the mix: https://pirate.black

You can audit Moneros math and source code, therefore you can audit the supply. It's more work than with Bitcoin to be sure, but it's still possible.

Monero already uses zero knowledge proofs, but in a different way than eg ZCash.

The opt-in nature of Zcashs privacy mechanism is not inherent. You can just flip a switch and disallow private transactions.