Ok.

Here it is:

This guys seems to be charging dealership labour rates.

no one flushes out engine oil. it is drained and replaced.

Euro cars tend to have soft discs and harder pads. not sure why. but discs wear quicker than pads almost always.

i think labour for rear disc replacement is too much.

the housing for thermostat, if genuine is ok price i think. but not sure on the design. gaskets tend to fail. not the whole housing.
Maybe you can't replace JUST the gasket, or he didn't bother to try because;

it was really hard to get to.

I am not sure where it is located. but 3 hours labour is a lot of money.

and last but not least, the coolant change is a BIG NO NO.

mixing coolant is BAD. over long term, it can corrode, leak, cause dramas.
Maybe VW is more resistant to this, but TOYOTA (MR REliable) suffers in the long term when coolant is mixed.

Get it flushed ASAP.

Any other questions let me know