According to this article
delivery of the first of three trains is due to be later this week, which suggests testing may be imminent (once the construction is signed off by the relevant Dutch safety authorities). The second train is due in mid May, with the third and final train delivered at the end of May. Hopefully, once testing has begun, we will get an opening date from the park - the article suggests the park has an opening date in mind but don't yet want to reveal it.
Also mentioned is the soundtrack for the ride. It's been composed by Efteling composer Rene Merkelbach and recorded by symphony orchestra Brussels Philharmonic in a major Belgian recording studio - it certainly sounds like the soundtrack could potentially be fairly impressive.
The more I hear about this coaster, the more I want to go ride it!