IMO the outstanding stable coin balance should be determined by a ratio: on one side the value of the outstanding coins; on the other non-Celo-based stable assets.
TradFi here in the USA for non-fractional (fully backed) banking and insurance financial products typically uses a ratio of 1:1.08 or greater, so they are all over-collateralized by close to 10%.
As to the choice of assets, my preference is to get as close to fiat backed as is possible (the less synthetic the better) for the time being and to choose at least three stable-ish coins.
In the medium term every effort should be made to vett and diversify into alternative assets like the investment pools at Goldfinch, Centrifuge, and others in that space that are tied to RWA’s.