Not sure what you are talking about... while the "overall" mystery of the "upside down" has not been fully explained yet (which makes sense as the series is not over) they have done a great job balancing the "close this door, open the next" strategy.

Every season had it's own core mystery that gets almost fully resolved... next season brings a new step in the chain and overall they have made it clear there is an escalation.

You seem to imply every season ends in a cliffhanger which I completely disagree with.

Side note, there are bad cliffhangers and good cliffhangers... i.e. Inception IMO was a good cliffhanger, you don't know if DiCaprio is dreaming or not but that is not really the important part, his journey ended one way or another, you get to decide if he "succeeded" the way you think "success" looks like.... Leaving the main plot without an answer, that is a bad cliffhanger