So the settings as they are will not help me:

Prices: entered with tax, displayed with tax


It would seem obvious that this setting should do the trick. However these settings just apply to products. When selecting “enter including tax” and “display including tax”, it will apply that to the products only. With both these settings selected. In the example from the post you sent me, they’ve set €5 for shipping, ánd selected “enter including tax” and “display including tax”. It will still add taxes on top of the shipping anyway:

Prices: entered with tax, displayed with tax (cart page)

So again, the goal is to set “enter including tax” and “display including tax”, and have it apply to shipping as well.

So if I enter €5 for shipping, and the tax is 21% – WooCommerce should backwards calculate that the shipping amount without tax is €4.13 and the 21% tax is €0.87.

If the cart contains a €10 item at 21% tax and another €10 item of 9% tax, then the shipping amount is €4.36 with €0.43 tax of 21%, and €0.21 tax of 9%. (The €5 being made up of €2.50 incl. 21% tax, and €2.50 incl. 9% tax – because the tax of shipping is divided proportionately to the tax rates of the items in checkout)