I've read all of your answers and they all sound quite reasonable/logical.
However, to spare thousands of dollars on something that really
wouldn't be worth it - to cut part of it off, why not structurally
support the tree by cabling and bracing it? It only costs a few hundred
at the max. You guys may be surprised, but oaks are very strong,
even if they do seem a bit rotted. But do contact a professional arborist.