You will need a rebuild. Or you should.

The trans has been failing for quite some time...parts have been worn and stressed, crap has been distributed. Getting a 'rifle shot' diagnosis of 'which bushing is bad' then only replacing that will be foolish.

When they opened it up and reported 'lots of contamination', that is the death knell. "Contamination" is actually "pieces of the internal transmission components"...

Sry
The ZF shops youve talked to should be able to do this. Labor is remove and reinstall transmission- then to do the rebuild. Just a guess, but here probably 5k-7k US$

How much money did you waste on the "BMW professionals"?