The reason why you can only do the ATPL skills test with an AOC operator is that, on the AOC they they are required to fly multi-pilot, for instance the AW139/S76 etc. An ATPL skilltest in the Netherlands is always combined with an OPC.
You won't be able to do an ATPL skilltest on an EC135 without an AOC holder.
If you meet all the requirements, find a job with an AOC holder and do they will do the ATPL skilltest with you (when they think you're ready)