Two part-time jobs or a secondary occupation?

Two part-time jobs where little income tax is deducted from both will indeed result in a lot of personal income tax to pay. But anyway, no more or less personal income tax than otherwise. It's not that you will ultimately pay more personal income tax with two part-time jobs at £20,000 gross per year, which together make £40,000 gross per year.

Than if you just have one job with a gross annual income of €40,000 from year to year.

It amounts to the same thing in both cases, the same amount of personal income tax. But in the case of two part-time jobs, a little less may be deducted in the meantime and you will have to pay a little more at the end.

But being self-employed does seem interesting to me. Then you can actually claim some expenses. But then I would dare to question for a little more than what you would otherwise get as an employee.

If you do it as a self-employed person, you have to calculate double what you would earn as an employee. After all, you incur a loss of about 50% in taxes as a self-employed person.

Huh?

But as an employee, you also pay 50% in taxes. Whether you do it as a sole proprietor or as an employee, the profit as an employee ends up in personal income tax, and the gross salary as an employee ends up in personal income tax. The taxes you paid in personal income tax are the same for both.