You can also operate the CO2 in a 'trans-critical' mode which gets around some of the issues of the low critical temp. This is done by using a gas cooler to cool supercritical fluid in place of the typical condenser.

There aren't really many good alternatives that are cheap, have very low GWP (less than 100), are non-toxic and non-flammable. HFOs are probably the closest to this, but even these are often mildly flammable and typically quite expensive (often > $50/kg).

Over here it seems CO2 refrigerators are increasingly common for supermarkets, warehouses etc. AFAIU it can be made about as efficient as HFC's, but the up front cost is higher due to higher pressures. OTOH CO2 is cheap, as you say, and there's no risk the next iteration of the F-gas legislation will make your new investment EOL.