Multiple levels of parallel stages in a Jenkins declarative pipeline
Sure it's possible to have nested parallel stages and it works just fine (I've done it before), but the visualization in the web UI is terrible, even with Blue Ocean.  There was even an official blog post when this feature was added.

This question has already been asked a couple other times on this site as well, you might be interested in these questions and answers:


jenkins pipeline nested parallel
How to build a complex parallel Jenkins pipeline?


It's not entirely clear to me from the wording of your question if it's a duplicate of those questions or if there are significant differences, so I haven't flagged your question as a duplicate.