I wasn't talking about UBI before but now we are.

The $12k is additive, but the factors that would reduce its value are multiplicative. If prices were to all double (an extreme case) it would still be equivalent to today's $6k. There's no scenario where everyone gets $12k but also the first $12k of everyone's money stops counting.

Sales tax on luxury items isn't a terrible idea, but the problem with relying only on sales tax is that very rich people (the ones who are getting most of the population's money) don't buy a lot, so the money just gets stuck in their accounts forever (or until they decide to buy all of the world's RAM). It can be one prong of a mixed strategy, though.

One part of inflation is driven by the difference between money entering circulation and money leaving it. UBI obviously brings a lot more money into circulation; it's generally assumed that taxes would have to be raised to remove the same amount.