I don’t think this is true

Actually, I think the links you provide back my points up pretty well. In fact, the Wikipedia articles you linked mentions that the Sovereign Grant is "15% to 25%" of the profits of the Royal Estate, not 25% to 35% as I thought. That's means the Government keeps 75% to 85% of their income, which is the "effective tax rate of at least 60%" I was talking about.

they own private residences... self sustaining not for profit...

Aristocrats gonna aristocrat. There isn't a stately home in the country that doesn't generate income from just existing. The National Trust owns a lot of them and is a charity that is broadly laudable, but most of the rest don't burden themselves with the notion of being "non-profit".

since 1993 the Monarch’s personal income has been taxable as for any other taxpayer

And as you mentioned, the rest of their income is taxed as if they were normal humans rather than aristocrats. It may be on a "voluntary" and incredibly malleable basis, but that's probably because they know they'd get crucified if they didn't and, regardless, the income from non-Crown Estate concerns is basically negligible.