Any digital centrally managed curreny will be at least as transparent as current account movements - might not start that way but difficult to imagine it ending any other way.
It seems nice allowing one to buy sex toys and such with pseudo-cash while preventing weapon-smuggling to do the same but I wonder if it's even possible in theory.
Think about interest rates as the rental price of money. When you put your money into the bank account you are basically a landlord for money. You must be on the lookout for people who want to use your money. Negative interest rates imply that there is a surplus and no demand and no way to store money (similar to renewables having a negative price when there is a surplus).
If you own a property but the market is so saturated that nobody wants to rent it, your property is worthless because nobody wants it. You're not supposed to build another unit and let that one sit empty too.
If interest rates are negative it means each additional unit of money is also worthless as the money is not being used for an investment in the real world. This isn't the same thing as the entire dollar being worthless. There is a portion that is invested and there is a portion that isn't. Inflation or negative interest rates reduce the uninvested portion until there are no more uninvested dollars. In other words, they are adjusting the value of each individual dollar back to its real value. This is generally why deflation or low inflation is bad. There is a risk that the perceived and the real value diverge and you see a huge crash at the end when they converge immediately. The good news is that persistent hyperinflation is unlikely because the problem is the exact opposite. The economy is running below its capacity and has room to grow.
The logical conclusion is that either you must increase debt artificially through the government as investor of last resort or we must prevent savings to exceed debt and thereby maintain savings = investment. A lot of countries actually refuse to do so, they chose a different route: Export debts to the US and let them deal with the problem. This is especially bad with Germany vs the rest of the EU. You aren't supposed to beggar thy neighbour. This is directly leading to widespread right wing populism in those nations. Just think about how unfair it is to shove difficult problems onto weaker nations that can't deal with them. The US is a hero from this perspective, it's strong enough to shoulder the problem.
It would be better if we had a wealth tax on money (investments don't cause macroeconomic problems so there is no practical need to tax them), we could then introduce negative income taxes and distribute the wealth tax revenue to people who will spend their money.
It's sufficient to have open (and trustworthy) protocols. Software will follow.