TreeD. I was poking around (sounding with the mallet and using a metal rod to probe the decay) in the cavity on the interior side of the large lead, it is over a single story, often occupied office space, the property gardeners had asked an associate of mine who runs a tree company to have a look at it and offer opinion, he in turn asked me to have a look. The result will be a 3 option report (couple pages) including reduce end weight then cable/brace, reduce end weight only, remove entire limb. Its pretty decayed, we didn't drill it but we figured there is a 3-5 inch wall of sound wood for about 70% of the circumference of the limb.
Removal of the whole limb would make the tree horribly unsightly and it would remove lots of foliage so we are both leaning towards option 1 with follow up pruning to reduce expected epicormics in a couple years and thereafter.
The building on the left is unoccupied so we didn't spend much time looking at the also decayed trunks.