Regarding the cost of the system, this hybrid model could have advantages for the Central Bank. As far as I know Tether works very much like a bank, i.e. they make profit via investing in different assets (among other things, Argentine agro-industrial companies ) and collecting the profits of these investments, gathering at least a part of the capital for their investments with their stablecoin.

In terms of efficiency, the Central Bank normally would like to realize this profit themselves. But Central Banks are very tightly regulated, so they can't invest in everything they like. A private company could do that, and thus the system could even be cost-neutral for the Central Bank and they wouldn't have to pay fees to the stablecoin issuer. One issue would be perhaps competition law: if another stablecoin company would like to enter a similar cooperation, the Central Bank would have a hard time to reject them because otherwise they would give an unfair advantage to a single company.

Regarding monetary policy, if regulation requires a high percentage of backing of the stablecoin in safe assets like bank accounts and bonds (not in stocks or other risky assets) then the Central Bank's monetary policy directly has a direct influence on the stablecoin's supply, even without a direct cooperation.

I guess however that such a cooperation could become dangerous for the Central Bank if the stablecoin company begins to struggle, for example due to too risky investments and other poor management decisions. It could be a "too big to fail" scenario, i.e. potentially the State would have to rescue them in case of problems. (Imagine if Terra/Luna in 2022 had entered such a "partnership" ...)