Would you care to explain why you think this is the most just?
Why do you think it is better than "I think it is the most just tax: you pay for what you earn, when you earn"?
Both sentences use exactly the same argument.
(For the record, I think that neither is the ideal solution, not least because neither takes into account that the value of an additional Euro is very different depending on your current wealth).
This discussion seems to me to be stalled because everyone uses the words "just" and "fair" while having very different definitions for them.
Do you mean Unreported employment or tax evasion by rich people?
Because both have different causes and would require different solutions.
Evading sales taxes is also much easier when you're rich, so I don't think that would help making the system more just.