Break the trip down into smaller chunks! You won't get sensible results trying to generate an itinerary from Netherlands to Turkey in one search.
Most journeys are going to involve getting first to Budapest, then on to Bucharest either with a very long day train or by the well-timed sleeper. Of course you can stop off somewhere en-route if you prefer.
Then (in summer) a direct sleeper from Bucharest to Istanbul.
You can use this London-Istanbul page as a guide. Only the dark blue route is possible right now. From Leeuwaren, you'd join that route in Germany or Austria I guess.
https://www.seat61.com/Turkey.htm
If you want to use another sleeper, there's one from Amsterdam to Vienna.