The problem is that the Guardian is selective in how it reports the facts so we have no balancing of views on both sides.
For instance they report...
the government treats it as entirely separate from the sovereign grant, the annual payment the royal family receives from the government to cover its official costs. That too has risen dramatically in recent times, and costs the taxpayer nearly £86m a year.
Which makes it sound like that the sovereign grant that the RF use to support their duties has risen. When in fact a temporary grant to cover extensive repairs to Buckingham Palacostsce has been giving to Palace officials who are overseeing the project. The money gets handed over along side the sovereign grant that covers the head of states usual duties but is not fir the use of the monarch.
The fact that the Guardian makes out it's all going directly to the King shows how they play loose with the facts. Both the queen and Prince Charles paid tax on the Duchies of Lancaster and Cornwall. There's no reason to think the King and William won't do the same.