You have pretty normal looking shadow detail, which means you didn't over-expose them.
The negatives are over-developed, verified by the extremely high density in the highlights.
Dektol isn't necessarily a bad film developer. You just have to watch it carefully so you don't over-develop. Try different shorter developing times, and I'm sure you will find a good compromise.
It is grainier than D76 for sure, but with modern films that could be a benefit.