We do have the same in Switzerland. In Switzerland you have to invoice using the regular sales tax, you can't deduct purchase tax and you have to pay a flat tax based on turnover.

For my customers I do configure products with the regular sales tax (but set to an income account instead of a payables account), with no purchase tax and you have to post the true flat tax based on the turnover manually. You have to configure tax codes accordingly.

Update:

I recommend you to create a new sales tax code for each tax rate which is applicable to your business (most probably you have only one for 20%). Name it "Flat tax sales 20%" as an example. Assign an adequate income account instead of an account payables account to your tax code (ask your accountant). You do not have to create new purchase tax codes, because you will not need any purchase tax at all.

When posting entries or configuring products, do always delete the purchase tax code assignation and assign the flat sales tax code.

When filing the tax return, just manually calculate the tax amount owned from the total sales and create the corresponding accounting entries (again, ask your accountant).