In the British constitution (and we do have one, even if it's not set out in a single document) "the Crown" represents what in other countries might be referred to as the State. It includes not only the Sovereign but also the executive (the UK and devolved governments), the legislature (parliament) and the judiciary. Most of the powers of the Crown are exercised by these institutions.

You don't seem to recognise the distinction between Charles as an individual and the position he holds. As an individual is indeed very wealthy (although not exceptionally so - he is only 263 in the Sunday Times Rich List). However as Sovereign he also performs an official role on behalf of the State, and the cost of this should be paid for by the State. These costs don't come out of your or my taxes but from income from property owned by the State.

You call for the Crown Estate to be nationalised. The Crown Estate belongs to the Sovereign, but this means the role rather than the person holding it. In other words, it belongs to the State, and to all intents and purposes has done since the eighteenth century. It is not King Charles's personal property. It is managed by an independent board who are accountable to Parliament, and its profits all go to the Treasury. In what respect is this not nationalised?

The State has overheads, which include the costs of performing the Sovereign's official duties. To repeat myself, the Sovereign Grant is not a personal payment to the King, any more than the £22m or so it costs to run Downing Street are a personal payment to Rishi Sunak.

You may consider some of these costs to be unnecessary or wasteful. That's a perfectly respectable point of view. However in the context of total public spending the amount is trivial, and even if the Sovereign Grant were to be abolished entirely it would make no discernible different to the public finances.