Hi Wilt,
now the case DISPLAY_PRICE_WITH_TAX = 'true' and Tax class = none is correct, thank you!

We tested the other cases again but now there is an error when we have:
DISPLAY_PRICE_WITH_TAX = 'true'
Tax class = 21%
product price net = 3.348 euro
product price gross = 4.0511 euro
We added 8 pieces of this product in cart.
Result: in the shopping cart page it shows a subtotal of 32.40 euro while if we check out, at step 2, subtotal shows 32.41 euro.

Best regards