Yeah,
600cc of oil in addition to what ever was there to start with is probably a big problem.... and may well be the cause for the increased resistance to run the compressor. This is not good and further operation may cause permanent damage to your system. I would have a serious discussion with the owner of the shop who did this to you.
Draining and flushing is a pain... you need to disassemble pretty much everything. But rather straight forward. I know some use isopropanol as solvent and compressed air with a very low dew point and absolutely no oil.... a paint shop is good place to talk to.... if there's no dirt in the system do not flush the compressor. Just drain it as per the manual.
I trust all this is explained in the FSM.
While at it you may want to swap the expansion valve with one which is calibrated to 134a.... and the hoses too - the old R12 hoses will let the new juice seep thru them. My 500 g charge lasted three years.... Your scope starts to stretch....
.
Good luck,
/T