I have. It looks clean.

I'll look down the TB ASAP

 

I had the same issue , almost 30 years ago with a small block 350 Chevy engine.. it ended up being the cheapest oil the local gas station had that magically disappeared.. being it was in a stepvan running daily I just carried extra with me.. filled it so often I never thought to do a oil change..true story.. the little things.. g/l
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I have found a lot of sediment in the engine block (from manufacturing) that settles about an inch+ think at the bottom corners of these blocks. That tends to trap the icky rust you are trying to flush out. You will eventually get back to green, but one flush isn't going to do it.

Something to be aware of on an abused cooling system like this is that the freeze plugs are galvanized steel from the factory and without the additional rust protection from an effective coolant, you will probably have rusty freeze plugs that will eventually rust through and start to fail on you. Sadly, the ones at the back of the engine require separating the transmission from the engine to get to. Now, don't lose sleep over this, just try to keep that coolant fresh from now on and that will slow down the process.