Not PP but I've been employed & self employed in Norway, with a similar system, and being self employed was a bit more work throughout the year, keeping up with invoices & receipts. You have to keep up to date for that as you have to send regular VAT statements. You need to use some accounting software, but if you're a small business you probably have that anyway. That accounting software has to integrate to the tax authorities and at end of year was just push a button to submit final numbers.

Then you wait for your updated tax return later in the year, which would automatically combine self-employed earnings, wages, investments, debts, tax deductions due to paying for childcare, etc... I normally have to spend 10-15 minutes to correct that if something like a travel calculation for my employer is wrong.