UBI would lower the effective tax, because it wouldn't put off everyone who might be able to work.
Denmark having high tax in general is another story, but at least with UBI not everyone who is getting paid will be thinking "hmm I lose all that income if I take a job, plus I can't be at home so much".
Realistically speaking, it sounds like introducing UBI would require a plethora of fundamental reforms (political, taxes, housing, job market) to be successful. While I enjoy thinking about that as a mental exercise, I consider it a complete fantasy - seeing how American politics work.
On the other side of the pond in a lot of European countries you already have this concept of "levelling", where progressive taxes control the higher end of the salary band to reduce income inequality. It's not such a large stretch to introduce controls on the lower end as well.
I would want UBI combined with a marginal tax rate that is much more constant as a function of income.
Current welfare systems, if you plot the effective marginal tax rate as a function of gross income (i.e. if you earn one more dollar, what percentage of that dollar does your net income increase including all government systems), have by far the highest marginal rates for the lower incomes. With the marginal tax-rate actually generally decreasing as your income increases. Notably in some broken systems (like the Dutch one) there can be cases where your marginal tax rate is above 100%.
Note that the tax burden ratio [(gross income - net income) / gross income] as well as the actual tax burden [gross income - net income] still increases with as gross income increases. If you graph the tax burden as a function of gross income, then the marginal tax rate is essentially the slope of this plot.
Importantly, it is the marginal tax-rate that affects a rational actor looking to weigh whether it is worth it work harder to earn more money. Hence very high marginal tax rates are very discouraging to working more / harder / trying to become more productive.
