And zcash would be better than Monero for privacy reasons.

How’s that? Zcash is opt in privacy, Monero is by default.

All modern Zcash wallets are shielded-only.

I assume a merchant would want a stablecoin more than Monero/zcash so that there's more assurance that the coins will sell for the same price.

citation ? (not doubting, just out of the loop)