For physical gold between 1mg and 0.5g I think the goldback is clearly the best (only?) option.

It's unclear to me whether silver can make a monetary comeback if it enjoys a free-market exchange rate to gold & we end fiat & fractional banking. If it does, that would reduce the need for minimal fractions of gold.

I think the 1/20 tz Goldback doesn't make much sense given that 1/10 tz coins & 1g combi-bar are available at such lower premiums. Do you imagine goldbacks circulating alongside higher-weight coinage? Or Goldback only?