As far as I’m aware, the payout should be 30% for taxes (unless you are in a country with a treaty, in which case it is lowered) and 10% (default) for Itch. I’m not aware of any extra charges, but I don’t know if donations are treated the same as actual payments. Could your payment method be charging something extra?
Also, can I still adjust the revenue sharing after such a long time? Might raise it a little bit higher because this is just a very great platform.
You should be able to change the revenue at any point without any issue. To do that go to “Account Settings” -> “Publisher” -> “Revenue sharing”. There you can adjust a slider for what percentage to give to Itch. I’m not sure if this happens at the moment of a purchase, or once you request a payout.