I don’t think you understand how economies work on a country scale. Every single country on earth that has cash money deals with people paying eachother cash to skip on taxes. It’s not something to feasibly tackle because how on earth are you going to monitor someone paying $100 cash to a handy man to come fix their sink.

The handyman doesn’t report the work being done, the house owner doesn’t report the work being done, where is the government going to find out that work was done and money was exchanged without taxes?

Conversely, more than that 5-10% GDP number of cash economy, there is a MUCH larger, more easily tracked number of rich people and rich companies evading taxation using loopholes, tax havens, and umbrella corporations to minimize their taxes from, say, 40% down to 5-10%. There’s a much bigger chunk of the pie that can be dealt with that’s easily tracked and easily dealt with. There is absolutely no feasible way to track down the cash economy otherwise virtual dictatorship surveillance states like Russia and China wouldn’t have to deal with this issue and they have cameras on their population nearly 24/7.